Seen as your such a fan of the 2.5, can i ask why your doing all this to a 4 cylinder?
I've got a 330d now, and comparing it to the 4 cylinder in my brothers 320d or my mates hybrid turbo Bora is night and day. The 6 cylinder engine just has so much more power and tractability, you can pootle along at 1500rpm and accellerate with no drama and it pulls really well from right off idle and smoothly thru the whole rev range. A simple remap and your closing in on 300hp too.
In contrast the 4cylinders are all dead ducks when off boost, with the power coming in as a massive slug somewhere north of 2000rpm as the engine finally comes on boost. As a result the rev range is really shortened and you end up with a "nothing POWER governor" affair. Fitting a massive 300hp capable turbo to a 1.9 is only going to make this even worse, probably not coming on boost until 2500rpm or more. My mates fitted a hybird to his Bora and really wishes he hadnt, because while yes, it makes 50hp more at the top end, its ruined the bottom end area where you spend most of the time. The old turbo was on full boost by 1800rpm, wheresa the new one is more like 2200rpm, and that 400rpm hole is exactly where you spend most of your time when driving normally. They're also rough, rattly and unpleasant sounding compared to the 6.
I've always been a fan of petrol turbos, and disliked diesels. I bought the 330d on a bit of a whim without driving it, and it really is bang on. If it was a choice between a 250hp petrol turbo or the 3.0 diesel with 250hp, i'd probably take the diesel. The same choice but with a 250hp 1.9 and i'd take the petrol engine every time.