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I asked a similar question regarding the M2 BB twins vs, a high-flow twin (Pettit/PFS).
For my application, I have no plans to exceed the stock boost levels, thus the M2 BBs aren't appropriate for my use, as they are intended for FD owners interested in making BIG horsepower.
The gist of the replies were that the M2s were suffereing from reliability problems with oil leakages.
Because my turbos have huge cracks in the manifolds, I cannot rebuild my current set of turbos. I must buy NEW.
M2 BBs are $2995. NEW JDM '99s are $2910 (Mazda Comp), and NEW stock twins are $2400 (Mazda Comp).
For less than the M2 BB twins, I plan to go with the JDM, Japan spec twin turbos. These are apparently more efficient and spool quicker than the stock FD twins (and as quick as the M2 BBs) because of the redesigned blades that are slightly smaller in diameter than the previous version. The abradable plastic seals help with reliability, and should last as long as the previous stock twin turbos.
I wont know the answer to your question for another two weeks. It will be at least that long until I can get the turbos on my car. the m2 set was my choice.
I had a very long conversation with someone at pettit and basically they said that there were very little gains to be had with the high flows...other than less cracking and such due to the ceramic coating.