Yeah, the stock manifold is pretty solid. Only reason I could see going with an aftermarket/custom manifold is if I wanted something to bolt up to a larger turbo. And even then I don't think there'd be enough demand to warrant producing one. There's still only a handful of folks on here going the bigger turbo route. There's probably a greater demand for UEL headers for the 2.5i...
Best bet would be to produce something that someone with a stock turbo could see benefit from. I personally wouldn't want a crank pulley, but it seems like a ton of other people do. Plus that would satisfy a few 2.5i 6MT owners too.