Re. crossover and performance: Ever see a crossover exhaust on a full race big bore XS650 motor? No? Ever wonder why? Re. back pressure being carefully calculated by Yamaha: No offense, but the term "back pressure" gets thrown around in ways that make me cringe, as in "His exhaust is popping because the motor needs more back pressure," when what is needed is proper fuel system tuning. There was careful calculation behind Yamaha's 2-stroke systems. As far as I can tell the only things that Yamaha calculated very carefully on the XS650 were the price point and the emissions compliance numbers. Re. fuel efficiency: I'm currently averaging ~54 mpg. with the following: 2-2 exhaust with 1.75" headers, Emgo 17" megaphones with straight through glass pack baffles, 77.5 mm. Shell/JRC pistons with static CR ~8.8:1, Shell #1 cam, and 33 mm. Mikuni flat slide 4-stroke pumper carbs. As to why I took up with XS650: Back in 1992 one of the partners in a local independent shop commented as follows regarding an F-model Special: "It's a cruiser and it'll never be anything but a cruiser." The bikes were cheap and much despised back then, and I bought one and started tweaking on it just to make that guy eat crow. And there were so very many things to correct in the way of XS weight, lousy stance (not talking cosmetics here), crap suspension, needless electical gadgetry, strangled exhaust and intake, and ergonomics that I never ran out of things to fool with. Peace, y'all.
And amen to that, Tebo! With road rage incidents becoming more frequent by the day, I'd invite the "louder and prouder" set to consider that an annoyingly loud exhaust can take your life by making you a target for some nutcase in a cage who's just looking for an excuse to kill somebody.