If you have your own car, you'd definitely make it:
(These are all worst case scenarios)
Show ends: 7:00
Out of HS: 7:30
Get to car: 7:45
Drive to Grand Floridean: 7:55
That gives you 5 min to check in
If you are taking the bus, you can still do it, BUT: you'll have to sit right by the exit, and ready to run out as soon as the show ends, so you beat at least most of the rush (a lot of people will be leaving HS after Fantasmic, so you'll have to be one of the first groups to the GF bus stop to be sure to get the first bus)
Show ends: 7:00
Get to bus stop: 7:15
Max wait for GF: usually 15 min: so, 7:30
Bus may have 1 stop before GF: arrive GF by 7:50
Plenty of time to check in by 8:00
Also, Disney policy is to horld your reservation for 10 min, so you really have until 8:10 to check in (though, they'll often give you more than 10 min, but it's not a guarantee)