A couple misconceptions here. First, the turbos are the same, but they have made improvements to the parts used. Which is very logical, cause a company doesn't profit by replacing the same part free of charge over and over. Plus, replacing the turbo on the MS6 isn't fun from the start.
2nd, the speed6 was never going to compare with the WRX or EVO, shouldn't be in that category. It's more of the luxury side with a performance attitude. Which is why I bought it. I'll get me a 2-5k whip to peep to my homies.

lol
As far as voiding due to mods, well that's no secret to anyone for any car type. Mazda is VERY light compared to Mitsu on their denials. Simply, if you give them a reason to question the use of the car/truck they do have every right to protect their investment as well. Just be smart about your actions. I'm very open with Brookdale in my plans for mods and racings. I asked them about aftermakret suspension. They directly said it would then void anything I'm covered on relating to my suspension, but if the motor went or something I'd still be covered. Adding an exhaust would of done nothing to my warranty unless I started messing with the turbo and manifolds. In most cases, just think cause and effect. And if you can, buy new from someone who has a warranty on their prodcut.
As for racing and such, I also ask first. If I am told an Auto X course will void my warranty, I won't do it, or I won't complain if I blow my car up by missing a gear or shift wrong leading to very high rpms. I knew what I'd be going into. Long story short, I'm covering my ass before I need to cover my ass, everyone else should look at it from their point of view "sometimes" ya know.
I'd say we have a couple kick ass Mazda dealers in the area, and I'd like to keep it that way, cause it only takes a few bad situation for them to shun the whole community.
And remember, if they replace a part through either 3 or 4 times, it starts getting into the lemon law.
And I havn't got my car in yet, so no update on my case. But to be safe, let you car warm up 15-20 min and if you see a cloud of blue smoke when you accelerate, you are leaking a bit though your seals, it's building up in the exhaust.