Sorry for the OT...
You know that you CAN see TAS/GS readouts on the HSI, right? (Actually as winds aloft are not modelled, I think it is pretty much the same) Not the same as a HUD toggle, of course, but it is there... IAS on the HUD, TAS/GS on the HSI
Now I'm confused... The only groundspeed I can read on the HSI (DATA page) is the prebriefed one from WPT to WPT, not the actual one; kinda useless since you can't read your TAS/GS anywhere else. Or can you? What am I missing? Who stole that page from my manual?
Besides that (not so important after all, anyway you'll always adjust your speed to match the prebriefed time to target or time to WPT), maybe winds aloft are not modeled, but air density is, and that positively affects to the gap between IAS and TAS.