First. the Veteran design (and or actual reg?) seem to be more tolerant than many other guns of adjusting hammer spring without the regulator and still getting consistent shot strings.
Second, and not mentioned yet here, is the matter of BCs. (AGN has a couple BC trolls that this type of discussion usually brings out of the woodwork so hopefully we can avoid them this go around. They are often the types who mostly shoot at 30 yards, which inherently makes their opinions not worth much when we're talking about taking pellets to 100 yards.) Anyway, in my personal experience shooting long range pdogs, long range paper, and the XFT matches, BC is proportional to wind deflection.
The 18.1s are a great pellet, and can shoot very accurate groups out to 100, as you've found. But also, as you noted, any wind and the 18.1s become a CHALLENGE. They usually have a BC of about 0.035, which only puts them a bit above the 10.34gr/.177 that is usually in the 0.028-0.034 range. And I've found the 10.34s only slightly harder to get to the intended impact point @ 100 (when compared to the 18.1/.22).
The 25.4gr MRDs have a BC up around 0.05, a BIG jump from the 18.1s. If you go back and look at the equipment lists that I include in the XFT match reports, you'll see mostly MRDs being shot, with the 18.1s having been attempted a couple of times. Now, the saying about the worst day fishing is better than the best day at work kinda holds true here, you'll have fun at XFT no matter what you're shooting, but past matches have shown the 18.1s to typically have one of the lower scores for that match. I'm not against the pellet, and I would love to see somebody come kick trash with the 18.1s. If you are very familiar with what they do in the wind, and can read the wind really well, you are likely to beat everybody, but if the wind has you guessing, it'll be a challenge. An enjoyable challenge.
if your Vet won't shoot the 25.4s as well as the 18.1s though, you won't have much of a choice. Shoot the more accurate pellet, even if it has the lower ballistic potential.