Gordon,
Well, I guess I shoudl chime in.. Before I begin, I'm no expert in turbos... disclaimer
I did a little research about Porsche 911 VTG system... And when i was reading it, this was one of my concerns.. I had to search a bit because I didn't know where I read it..
Quote:
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Variable Turbine Geometry has been used extensively in turbodiesel engines since the 1990s, but it has never been on a production petrol turbocharged car before until the new Type 997 Porsche 911 Turbo. This is because petrol engine exhaust gases are alot hotter than diesel engine exhaust gas, so generally the material used to make VTG turbos could not stand this heat. The 997 911 Turbo uses a BorgWarner VTG turbocharger which uses special materials derived from aerospace technology, hence solving the temperature problem.
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Well, we know rotary engines are hotter than norm, so, not sure how this turbo system will last... It might but not sure..
Now regarding sequential system..
I think manifold design is crucial! If you look at your 3rd gen manifold, you'll see where some of the faults lies. Some lack of sequential problem could be the cracks on your stock manifold. I kind of wished Mazda would of made it out of cast stainless steel and like Garfinkle said, bigger door and such.
Next issues I see it the controlling of seq. system. Most are driven by vac. and solenoids. Another solution might be variable electric motors to move the actuators instead (Maybe??) Controlling turbo and wastegate thru fast acting variable motor might give better control over turbos.. Again, I'm no engineer so this is purely pulling it out of my ass.
Next, turbo technology. Back in the days, we all thought bigger is better. I never when with large single because I prefer less lag vs. more

I wasn't about POWER!! but just something to drive and enjoy. Now, these mid size ball bearing spool as well as stockers. I think combination of couple of small ball bearing turbos, good manifold for sequential set up might do the trick...
Again, I'm just an enthusiasts and just an idea..