"If FMC target descent speed was 280 and it took 300 to stay on path, would I get the message?"
No, you wouldn't get the message until approaching Vmo. However, in years of operating the aircraft, I've never gotten much outside of the programmed speed +/- 20kt. At target speed -10 I adjust thrust to stay in HOLD, and if the speed is uncomfortably high, I start to use a little speedbrake (often there is another constraint ahead). It really isn't an issue.