What is the top speed if the limiter is removed? Thanks.

Since you're apparently a US resident, this is somewhat theoretical.

It's important to consider that some cars are geared ultra-tall in top gear - for instance, LS1 Camaros, with a 3.23 or 3.42 rearend but the T-56 transmission's sixth gear is a 0.5:1 overdrive, for an effective rearend ratio of ~1.7:1.

The M5 is not like this. You can remove the 155mph limiter, and the car will run to maybe 185, but at that speed the engine is hard against the redline, which is not good.

I'm happy with the car as-is on the gearing front with its 3.15 rearend, certainly for US use where hitting 140 is rare, but if top speed were the goal I'd try to go to 2.80 gears.