I know I'm in the minority on this issue but here's my opinion. If you can do 'em, and you're offering them at this relatively minor price increase, I'd want it on my barrels. Actually, and to be completely frank about it, I've got quite a few barrels that I'd like to have done but I've always been put off having them Freaked because of the lack of o-rings on this kind of custom job. Ever since I'd tried to use my WWA freak barrel without the o-rings (and suffered for it) I've always seen the need for them.
How about offering it in another way? Like putting the o-ring groove(s) in as standard but give everyone the ability to 'opt out'. I know it still amounts to the same thing but having 'options to drop the price' always sounds better than 'extras to increase the price'.