Flip back rest forward, slide seat as far forward as possible and its comparable to stock seats. Adults havent had any issue, kids definitely dont have an issue, but I guess its relative.
I usually dont have passengers in the back. Back seat is out most of the time since I use it as a truck most of the time when not wheeling. Only person I usually have in the back is my 16 y/o kid if the Mrs. is with us...but Mrs. is 5'2", she's not a fan of climbing in and out of my rig too often....lol. For me, its worth the extra comfort. If your putting a car seat in the back, I would imagine its not going to be easy whether stock seats or aftermarket.
There are a couple folks on here that have made brackets so their seats hinge at the front and flip all the into the dash which is a good idea. I believe MrK5 did it on his old K5 before he switched to a crew cab.