If HTC continues on its current trend when assigning top speeds of AH gv's, the M18 Hellcat will have a 50mph top speed.
Do people realize that the M8 Greyhound had more armor up front than the M18 does??? The most armored place on the M18 is the front of the turret with 44mm of armor (25mm on actual turret and 19mm from the mantlet). Otherwise the front of the hull of the M18 had 13mm of armor. Yes, I said 13mm. The M8 Greyhound had 16mm.
So basically, the M18 will be an M3 (M3 = 48mph) running around with a big gun. While I'll vouch for any new gv that will fill a gap (the high mph/hvy AP firepower slot is currently void), a remodeled M3 and corresponding 4-5 variants might go further.