The GSXR600 throttle bodies will outflow the XS engine easily. The GSXR600 uses 4 38mm throttle bodies, people have used those throttle bodies on 2l car engines and push around 200hp. Really it would be better to have 32-34mm throttle bodies, it would smooth the off idle and low throttle movements out. The 38mm throttle bodies will flow more air than a 38mm carb most of the time, no slide, no needle, just the butterflies to disrupt the air.
Microsquirt is one of the cheapest smallest ECUs out there. I paid a little over $300, straight from DIYautotune. Problem with adapting an ECU from something else is you have to replicate its trigger signal, buy a piggyback like a power commander, and hope that gives you enough adjustment. So by the time you get a $50 used ECU working you are easily above the cost of a microsquirt.
So far for the EFI swap it has cost:
Microsquirt- $330
Sensors- $50
GSXR ITBs- $45
fuel pump- $30
misc part- $100
So I am about $650 in, and really last couple things are make the fuel pump mount, drill a new fuel rail, and finish wiring. So about the same as getting a pamco, and good aftermarket carbs.