Having higher speeds than standard FreeSpace isn't a problem, just look at Dimensional Eclipse
(totally not a shameless plug) or, if you want to go a lot older, there was the Velocity Mod for retail FS2. The problem isn't the speed itself, but the relatively decreasing play area the faster you go. There's a very specific range from the center that the game will just kill you outright if you go too far. It's there since floating point calculations would start breaking down beyond that distance.
Dimensional Eclipse's failed spiritual predecessor, Stellar Assault FSO had speeds of capping out at around 600 m/s. DE caps out at 350 or so (except for one fighter which goes up to 600 m/s). I will say that it would feel really, really weird for the FreeSpace universe to suddenly get drugged upon on some really strong speed.