Yes, it's possible to do both. To change the tempo of an SPC file, you'd need to either understand the underlying music format to change the note lengths, or edit the underlying assembly driver code to play everything faster. Each game can be different, but typically games made by the same company use the same or similar format and engine. It's probably more work than it's worth.
Changing the samples is a matter of replacing the BRR samples with one of the same or smaller size. If you have a bigger one, you'll most likely have to shift all the other samples and change the pointers to them.
Another idea is to see if any SPC players allow you simply to clock faster and speed it up that way.
Lastly, it's also possible to convert an SPC to MIDI and then you can play it in your favorite MIDI and play with the tempo all you want.