You can get away with it, for sure. Would the tightness of the sound be ideal? Not really. I've played the Schecter 8 string Hellraiser before and felt the tightness of the low F# string tonally was sub-optimimal.
As it is, a low A string on a 25.5 inch scale guitar is pushing things, so I honestly wouldn't go any lower than G# on a 26.5 inch scale.
If you wanna go to F#, you should seriously think of getting at least a 27 inch scale guitar, but preferably an even longer scale