I don't think there are any special rules for "if it/that were possible". Syntactically, it's just an if clause. What's special about it, at least in Frank Deford's sentence, is that it's not being used literally. He's using it as a figure of speech. If you understand how the figure of speech works, you'll be able to use common sense to decide when to use it and how to use it appropriately—that is, what else in the sentence, if anything, you need to vary to "agree" with it.
The word were is past subjunctive. With if, the past subjunctive literally means that the if clause is not assumed true; the sentence merely describes a consequence of a false hypothesis, like this:
If wishes were horses, beggars would ride.
Source: Well-known proverb.
The point of this proverb is that deducing happy consequences from a false premise is worthless. It expresses this by pointing out that if you accept the absurd premise that wishes are horses, then even very poor people (who can't afford horses) would be able to ride them. Sometimes people say this proverb to counter someone who's treating a fantasy as a feasible plan. "If we won the lottery, we wouldn't have to pay rent because we could buy our own house!" "Yeah, well, if wishes were horses, beggars would ride." The past subjunctive were evokes a feeling of entering into a hypothetical world rather than reality. People use it mainly to create a strong feeling of distance what the clause says and what they're claiming to be true.
Literally, Deford's sentence means that it's not possible that Blue could have gotten an even greater early lead than in the first race. It's like saying this:
And this time, if two plus two equaled five, then Blue reached out to an even greater early lead.
In other words, the literal meaning is that Blue could not have outrun Gavin even more than before; it's a consequence of an impossibility, like "beggars would ride". Literally, it doesn't make much sense: "If it were possible that Blue outran Gavin even more, then Blue outran Gavin even more."
Of course, that's not what Deford really means. He really just means this:
And this time, amazingly, Blue reached out to an even greater early lead.
Using "if it were possible" as a figure of speech here just conveys a certain kind of amazement or disbelief.
Normally in this kind of sentence with if, the main verb would be in the conditional mood: introduced by would, could, etc. But in this sentence, the main verb, reached, is in the indicative mood. That's because Deford doesn't really want you to treat the sentence as an absurd consequence of an impossible condition. The point is, Blue did get an even greater early lead. If Deford said would have reached, the reader would be confused. The reader would wonder if Deford might really only be talking hypothetically. The figure of speech would be spoiled if Deford treated the grammar too consistently.
That should give you some insight into how ordinary grammatical rules apply to this phrase—or don't apply. You have to reasonably bend the rules to keep the figure of speech comprehensible. It is, after all, just an extravagant way of saying amazingly. You can use it in any context at all where you want the peculiar "impossible-hypothesis-as-reality" sort of amazingness. You have to judge separately in each case how much the figure of speech diverges from ordinary usage, how likely it is to create confusion, and how or whether to adjust other grammatical elements to prevent that confusion. For example, here's a rather different figurative use of "if that were possible":
I knew the Duke of Newcastle to have extremely clean hands, maybe even too clean, if that were possible—for, after all the great offices which he had held for fifty years, he died £300,000 poorer than he was when he first came into them.
Source: Edited from a letter by Philip Stanhope, 4th Earl of Chesterfield, Nov. 21, 1768.
The idea here is that the Duke of Newcastle was extremely honest ("clean hands"), maybe even too honest—if that concept made any sense, which it doesn't. But in a sense, the Duke was "too honest", because, rather than merely refraining from corruption, he actually seems to have lost money while he held high offices in government. It's figurative speech. Notice the contradiction between "I knew…" and "if that were possible": of course it's possible, or the author couldn't have known it to be true.
Regarding your own use of "if that were possible", I concur with cjl550 that the word that is appropriate because the fact pretended to be an impossibility has already been stated. People also say "if such were possible" and "if such a thing were possible". And I concur with cjl750 that it would be customary to include the word even before more, to agree with the idea that the previous craziness was already very bad—but it's not absolutely necessary. I think the figure of speech makes more sense, though, without the past subjunctive:
So I tried to fix it but I failed; it started acting up even more crazily, if that's possible.
The conceit of knowingly entertaining an absurd hypothesis doesn't work here, because the context hasn't provided a reason to think the hypothesis is so absurd. The level of craziness seen so far isn't comparable to wishes being horses or a person being too honest. Putting the hypothesis in the indicative mood here, "if that's possible", is more like wryly rolling your eyes as you're speaking rather than signaling dumbfounded amazement as in Deford's sentence. But even this weaker version of the figure of speech might be too strong for a computer acting up. We all know all too well that that's possible.