Yes, there is a need for the crossover tube. I'm not much for explaining the whole deal behind it, but without a doubt, if I ever go back to dual carbs, I'll be using the Pro Design Intake and it has the spots for a crossover tube. I've seen dynos with just a change from stock to PD Intake and there's a nice increase in performance. I've heard the Boss intakes are good as well, tho I've never seen dynos showing the added power.
I wish I had the money to experiment with carb sizes also. I'm sure Trinity did some testing before they offered the kit to the public. If I hadn't sent my cylinders off for porting, I'd consider trying a 33mm. I think with the stock setup, the 33mm will work better. More velocity would mean more vacuum for the fuel and I think it would give better response, and make it easier to jet. I don't necessarily think the power will be a whole lot different with the 33, I just think the air velocity has a lot to do with how a carb performs and a 35mm carb is a big carb. With porting, it'll be flowing more air so there's the increase in velocity.
Some people say why not just use a 38mm or something for a stroker, but at mid throttle or low rpms, there's less need for airflow and with a 38mm or bigger, there won't be much velocity at all and I think the power will suffer. I might try it one day. I'd still like to experiment with a 33 as well, and I just might!!
Good luck on your carb decision.
I had the 35 on and switched to the 38 off my 250r, it has a little less on the bottom, but I gained 5 mph on top. It definitly pulls harder. I have only bolt on parts no porting.