That’s massive. If you can do 370 W for 90 minutes, I think an FTP of 400 W (370 W = 92.5 % of 400 W) is much more plausible than 390 W (= 95 % of 390 W, i. e. threshold).
For a guy as “heavy” as you, these numbers would not be good, they are spectacular. To give you some perspective, world-class sprinter Marcel Kittel peaked at 4.9 W/kg in his career. Yes, he was more powerful, but he was also a few kg heavier than you are now.
On the flip side, aiming at 5 W/kg might be out of reach. But I think you are just setting yourself the wrong goal.
Are you nuts? I wouldn’t try to lose any weight if I were you. Whatever you are doing is clearly working for you! 
Cutting weight could be very risky for your training. I’d continue what you are doing now and just race according to your strengths. I reckon few if anyone in the field can match you in terms of raw power. Even in terms of W/kg you’d like be at or near the top of the peloton. Plus, raw power helps offroards, because you need to overcome friction between tires and the road surface.