Good stuff. :) I'm digging that alternator relocation... cleans it up nice. We're putting an Elite 1500 ECU in my buddies right now... just got tired of the PowerFC life. This is my first full dive into Haltech stuff, and so far I like what I see. His is set up for about 19psi on that new S300SX-E on a Howard Coleman Motor, with an occasional dab into the 25psi setting... it's a riot. We're also building a 2" SS schedule 40 manifold for it right now... it builds a lot like the land speed turbo harley motors I've been working with for the last year. :)
Back on topic: 8-10 is doable on a stock motor, I did it for a while... but 10psi is enough to tie the stock chassis in knots; you're going to need good boost control to be able to really ride it. I absolutely love WI/AI, but you're right about the added complexity. It's been a long, hard road to get a reliable system on mine, it takes up a lot of room, and it's really hard to plumb with an A2W intercooler core in the airbox. Honestly, if you intercool it first it's just a lot easier to pour in C16 if you want to do big long pulls on more than 8psi than it is to mess with the water stuff. I'm still going to use/build it for Bonneville stuff, but we're talking 3+ miles of wide open out there. For sane street riding it shouldn't be an issue... tune conservatively!
As far as 'drop in' injectors go, you can use other model year injectors, but you only pick up 80-100cc/min with those. If you ever want to see more than 230ish HP you're going to need more than that. The ID's really do 'drop in' easily (see this thread/post for what it looks like on a second gen set of throttle bodies https://www.zx-10r.net/forum/3088626-post40.html
)... making those spacers is really easy (you can just sand/file/grind down generic spacers from the hardware store if you don't have the tools to make your own). Aside from that, you can make or buy pigtail adapters if you don't want to modify the stock harness. I'd be happy to make you professional PNP adapters at the cost of parts/shipping if you went that route. The first harness I did the ID's on I de-pinned the stock connectors, cut and re-crimped the new connectors for the ID's on and it worked great; no splicing to be had! It's nice that ID supplies the pins and connectors at no cost with their injectors.
A 255 pump is ok, you just have to do something about the fuel filter set up. If you just drop the pump in the stock housing it will eventually end up in tears. You can change the pre-pump filter (sock) all you like, but the actual filter is built into the housing and cannot be serviced. I've built my own housing with a serviceable filter, but it was a pretty big project... using an external pump with a serviceable in-line filter is a much easier way to go. If you absolutely must use the stock housing/filter, put a fuel pressure sensor on that bad boy and build in a failsafe... it's only a matter of time before it causes a failure.
I really like Turbosmart's FPR800 series (I currently run that one on my bike), or Fuel Lab's 535 series (53501).... the Fuel Lab regulator is freaking tiny, and very nicely machined! A 1:1 regulator is what you want... it's tempting to try the adjustable rising rates, but you want to keep fuel pressure as low as you can, and wildly varying fuel pressures (especially high fuel pressures) cause a host of other problems.
That rapid bike stuff honestly looks like a step in the wrong direction. If you are going to stay stock ECU, the PCV has all the capabilities you need to add boost trims (minus a MAP sensor clamp, which you can just check valve), costs a LOT less, and is likely supported/familiar with a LOT more tuners/dyno operators. That being said, a full stand alone is the best choice if you can do it. I switched to a microsquirt first and was thrilled with how well it worked, and then went to a MS3Pro... the control you get and tight packaging is perfect. You'll end up wanting more data and datalogging than most piggy-backs can ever provide, and you're going to start running out of room with a quickness if you plan on keeping it street legal... stand-alones are really the only way to go if you want to be serious about performance or tuning. Now, if you're happy with a weekend wheelie machine that is just fun and not necessarily competitive, the piggyback route is fine.... but I sure as hell wouldn't pay 800+ dollars for that Rapid bike set-up, even if it was backward compatible with the first gen.... I bought my MS3Pro for that price! Shit, if you add on their quickshifter and AFR/closed-loop stuff it lists over 1300!! Nucking Futs.