Anytime you increase the size of the tire/wheel combo, you make an impact on the final drive. Yes, it will have an effect on how "peppy" the FJC will feel. Your also adding some significant weight to the rotational mass, that will impact it also.
The question is: how much? You are going from a stock tire under 32" to almost a 34" tire, taller and wider. I "felt" a difference on mine when I went from stock to 255/85/16's, IMHO, you'll feel it going to that tire. You'll also have to deal with rubbing on the front body mount.
I love the BFG KM2 tires, run them on my FZJ80 and are a very nice tire on the road, even better off-road.
Oh, welcome to ih8ud...