FLYBOYJ
"THE GREAT GAZOO"
The top speed of the basic 187 was about 330 mph as flown although one was flown at 395 mph which I could assure you weren't close to a capable combat aircraft in hat configuration. Just because you put more powerful engines on board doesn't mean you're going to get great advantages in performance, especially on a twin engine aircraft.However, FW-187 had 1400HP onboard, against 2200 early Bf-110s had. And 110 with almost 3000HP was not capable for 600km/h in clean configuration.
With DB-601E (same as Bf-109F4 had, for example) on board, the power is almost doubled for FW-187. My guestimate is that speeds well beyond 650km/h would've been easily achievable. Not bad for 1941, with off-the shelf parts.
The 187 would have cost more than the 109 and 190 just by virtue of its configuration.