The Quest doesn't use a common feed neck thread, and to the best of my knowledge, the Critical feed neck is indeed the only after market feed neck available.
The problem with trimming the top of the stock feed neck is that there is a rim there to hold the clamping ring in place - trim the top and you lose the rim. If you were careful, that shouldn't really be an issue I suppose.
Thing is though that the rim is very small anyway, so you wouldn't gain (or lose as the case may be!
) much by removing it. That would then mean needing a narrower clamping ring so that you could shorten the feed neck more without the ring getting in the way.
The Critical feed neck sits a little lower at the base, and has been shortened quite a bit at the top. With a short loader neck, your loader can sit pretty low. You also have the bonus of a lever clamping feed neck, rather than just screw clamping.