It of course depends on where you sail.
When Surf tested FreeRace sails (like Gaastra GTX) in the size 8.5 they found that the Glide 7.5 did have more low end (both planing and speed), but on the other hand it had less top end (both speed and control).
The conclusion was that the Glide 7.5 is an excellent sail in places with light and stable wind, like where winds are thermal. In places with gusty and variable wind the Glide was thought to be too much of a wrestling match.
But it does prove an intersting point, there is not a single truth when it comes to sail size!
If the Glide 7.5 has more power than a freeride 8.5, then a Glide 8.5 has to be VERY powerful. It would be fun to try it...