I only see a few fast twist 22-250 being offered but I did see browning offering a left handed 22-250 with 1:9 twist. I have a fast twist 223rem. Would fast twist 22-250 bring anything more to the table in practical term? ie make a better deer round over 223rem?
I'm not sure why you're asking about deer on the varmint/predator part of the forum.
At .22-250 speed, the fast twist has tradeoffs. It won't work as well with a number of varmint bullets I've used over the years. Twist is too fast for the jackets. You will gain ability to shoot heavier bullets but I'm not sure they're very useful for varminting. A buddy built a .22-250AI with a 1-9" twist. Shooting 75 grain A-MAX bullets he was getting mixed results. Accuracy was good, but terminal performance was considerably variable. Some would blow up as you'd hope, others would pencil through.
For varmint shooting, I want bullet disintegration on impact, partly for clean kills, partly to avoid ricochets. The fast twist gets in the way. If you can find them, the 53 grain VMAX partly solves the problem .. it is tough enough to handle the RPM and has a pretty decent BC especially for its weight.
For deer, unless you're shooting bullets 70 grains and up (I do like the 70 grain accubond), you don't need that 1-9" twist. A 1-12" will handle most hunting bullets .. not all. It will allow more flexibility in your varmint bullet choices than the 1-9" twist as well.
For varmints only, I'd stick with the original 1-14" twist. For varmints + deer, I'd go 1-12". For deer only, then I'd consider 1-8" or 1-9". For long long range paper I might even consider 1-6.5" or 1-7". Maybe. I would not build a dedicated deer gun in a .22 caliber. There are better choices that don't involve quite so much compromise. IMHO of course.
Tom