My two-penny's worth on the manifold conversion on M52TUB28.
I have added the M54B30 manifold & DISA Valve (replacing the PCV with an oil separator at the same time), sorting out all the vacuum lines with new, ebay adaptor plate and standard M52TUB28 throttle body plus a re-mapped ECU to account for these.
The difference? idle to 3000rpm - a little bit more torque but not huge so goes much like a standard 2.8. Above 3000 rpm - "OHHHH MYYYYY GODDDDD!" The acceleration is relentless right up to the redline, the car nearly takes off.
Recommended mod? - Absolutely. This is, without doubt, the best "bang-for-your-buck" on a 2.8 double vanos car.
If you add in the following, which are also on my car, the difference is even more dramatic:
1. Pipercross foam cone filter (one for Vauxhall VXR - so large volume of air possible), make sure it gets plenty of cold air.
2. Cat-back custom twin 2.25" Stainless Steel Exhaust with quad tips (no silencers, original cat and resonator retained).
3. Delete Viscous Fan and add electric.
4. ECU re-map with: 7200 RPM limiter, Delete Secondary air pump, delete rear O2 sensor, mild tune (stage 1 ish), tune for the free-flow exhaust, M54B30 manifold and cone filter.
Not Done yet
I have not gone as far as a Cam upgrade.
I have not added a long-branch SS Header/Manifold for the exhaust, partly due to the lack of RHD items available (although I may have found one yesterday that fits), partly because the standard item seems to manage all the extra airflow OK but mainly because of the clearance issue with the steering column.
The main reason I have not yet gone further with the above bits and further tuning, is that the car is only 1150Kg (ish). This combined with the extra air-flow and fueling allowed by the conversion means it absolutely flies under full-throttle and has a very nice balance of power, agility, and feel. any more power "might" be overkill - but then again!!!!
Easy Job? Nope, the manifold is a PITA to get in and out mainly due to the amount of Macaroni (vacuum, PCV tubes). Most of these I have deleted/re-routed via an oil separator/catch can set up which works really well and does not shove all of the mustardy, oily crap down my inlets (clean valves), and deleting the Secondary Air Pump.
If you need any help drop me a line. Good luck.