How is the performance rating on the Mazda 6 5-speed trans ? Will it accept more power than the P5 trans ? Will it fit in a P5 without modifications ? Would the V-6 trans fit the 2.0 found in the P5 ? I have heard the P5 5-speed is not horsepower friendly What is the max power I can safely use before the Trans says F@#$ this I'm outta here ? I want to be around 200 whp by next summer.

 

It will accept more power than the P5 tranny, as the same tranny is in the I-4 and V-6 variants of the 6. However, the story is basically the same as your engine question... its' an all-new tranny for an all-new car, and it shares NOTHING with the G15M-R tranny in your car, so this is NOT a bolt-in swap.

How much power will the G15 hold? Depends exactly how you intend to achieve your 200 fwhp, because horsepower does not breaks trannies, TORQUE does. Mazda has never installed a G-series in a car with an engine rated much higher than 180 ft-lb of torque at the crank, so you can use that as a hint. If you install a turbo/supercharger that cranks out 200 fwhp, you will probably be right around this limit; N/A mods will give you a bigger margin of safety because they will not significantly increase the car's torque output- you will probably not exceed ~145 ft-lb unless you use a c/r that's unreasonable on the street.

My thought: you will probably be pretty safe AS LONG as you drive gently. NO 5000-rpm clutch dumps, powershifting, hard shifts, or hard acceleration on broken pavement. I'm personally convinced that many of the problems with P5 tranny breakage reported on this forum are related to abuse, but YMMV.