we were just discussing this on another site.
grab the pipe and arrestors for $200, that's a steal IMO
add ECWI, best bang for the buck IMO
never tried a tdr myself so I have no comment except that I personally own a very very noisy yamaha with free flow exhaust and the drone gets real old real fast.
yes, carb rejet will be in order.
hard to catch the GSX with an X4, (xo prop would help) without sacrificing low end , i'd rather just be faster than he is out of the hole and keep all races under 200 yds and you'll never worry

a decent running stock GSX951 will top out around 62 ish, and although the X4 is capable of passing that, its not cheap to do so, but getting you to 59ish, and getting you there fast isn't that expensive, but those last 3-4mph don't come cheap past 59, again, XO helps and its not quite as bad on the low end as you might think, especially if your pushing the rest of the ski with some serious mods, it will yank your arms hard.
I liked the low end response of the swirl prop myself, but a good stocker would be my 2nd choice (others will chime in with opinions on what might be a better fit) but I also did the ECWI the same weekend as the swirl and the difference was night and day, huge improvement in midrange throttle.
i'm not a fan of sponsons, i like the controlled slide turn of stockers, but i'm a rec rider, not out running bouys. I've ridden both and i like spinouts and 360's (got pretty good at them) and with beach house, that's not happening, they grip hard, no sliding involved.
I never liked UMI, rode with and w/o on X4's and IMO they were a slight improvement, but not drastic, and I didn't like that if I was cruising at 45mph that there was no play in the bars, every little movement of the bars moved the ski, and I didn't like that, couldn't' relax when cruising. they are nice, and asthetically pleasing, but not worth the $$$ from my thoughts. (others disagree totally with me on this) A much cheaper option to UMI is stock stem, aftermarket bars, finger throttle and new start switch.
I believe I was running around 58.6 with the swirl, with great low end response, and about 2mph faster on top end with XO, it got there fast. I had Factory 1, proks, r&d nozzle, swirl, ECWI, hard jet box, all else was stock at least performance wise.
I can't stress ECWI enough, it really pulls hard, and will give you that extra 2-3mph over that last second when throttling up to a wave for a jump. maybe consider hard jetting the waterbox as well. downside to ECWI and hard jet is you gotta watch those jets, and check them often, since a clog will cause some issues, but I rode in sandy/salty conditions for maybe 18months with it, just checked them every 3 weeks or so, and never had an issue myself.