It's acceptable in some of the cars, like the GTR cars, the Formula cars and maybe the FZ50 as well. But still, I flat-shift in all cars, because I don't have a clutch-pedal nor a shifter, so I really cannot be bothered with lifting off.
I say "maybe the FZ50" because in my mind it's a bit like a Ferrari 360 or a 430, and those two cars both have a sequential gearbox with paddle-shifting and an auto-clutch. There is no need to lift in those real cars, because the automatics clutches so fast that you'd have to lift completely for just a milisecond, then hammer the throttle again, which is impossible to coordinate.
In the other cars though, (XF, XR, LX etc.) it is more realistic to lift when shifting, but then again, if you don't have either clutch-pedal or shifter it's pointless.