What custom Yoshi exhaust do Team Classic Suzuki and Team Hammer use on their bikes? Seems they use Team Factory Suzuki Yoshi stuff.
One issue is that to homologate the 750 for racing, a drive by wire throttle assembly and an opening limit of 78% was imposed for closed course stuff. Your engine maybe has a throttle cable and that is not a concern.
It's interesting that the yosh exhaust pn is the same for both the 600 and 750...
Leo Vince exhaust with some aftermarket intake stacks might be worth a look?
Maybe look at the gsxr 600 exhaust for ideas as that combo seems to have a broad top end without falling on its face at 15k.
Lees Cycle Service in San Diego may have some race gixxer 750 information.
Okrapovich (Akrapovich) has a pdf of their racing exhaust - they use tapers and may bigger diameter secondaries...?
You've already modeled the 750 with the previous modded 600 exhaust - anything interesting there?
There are a few threads on the gixxer forum but the links to dyno graphs for hindle, yosh, and okrapovich are all junk.
Are any teams using paddle shifters? Human factors would lead one to believe that reducing the amount of hand/arm motion on the current shifter would lead to quicker shifting, hence more "on power" time for acceleration. That would need to be evaluated by the driver but it appears most driving is one-handed except for the really tight turns.
Your call; go down the exhaust development rabbit hole (where 5 or 6 aftermarket companies have already BTDT) or help the driver be more efficient. The real challenge will be how the team supports the kart after you're done with improvements. What's the schedule for top end renewal since banging off a raised rpm limiter will be commonplace? Maybe Spintron the valvetrain and find the golden valvespring that promotes a little loft?