I just switched my boys around to be forward facing (right before their second bday). They never seemed uncomfortable as far as their legs (I have one ds who hates straps, but that is still true facing forward).
I think the safest thing to do is to leave them rearfacing until the limit of your seat's weight allowance (mine was 30lbs and when I checked my boys weight they were 25 and 33, so I switched them both). I know of a lot of kids with my longer legs than my ds's and they just figured out ways to cross them, or bend them and seemed fine.
I think its the development of the neck and spine that is the concern, that the motion in an accident has much more support rear facing.
The downside to forward facing for us now is that the seats are so upright that they have trouble sleeping (we used to always plan our drives during nap time), but of course now they can see the "world around them" and have a lot of fun seeing things (not to mention seeing us easier - but we had mirrors on their seat backs before so they could see us).
I know that singleton parents complain also that it bothers their kids to be facing backwards and not seeing them, so in that way I probably had it easier since there were two of them facing the same way, they could see each other, etc, it never occurred to them that it was "the wrong way"
Here is a link with lots of good info:http://www.car-safety.org/