I would have expected gm midi mappings - C1 for Bass, D1 for snare, hi-hat on F#1 etc
After rereading more carefully this part of your post, I have to say this:
The way Spark's 8 pads are presented, on 1 horizontal row coupled with the current PAD-to-Instrument assignments are and feel more natural while playing them through the Pads and not so through the Keyboard (hence, your post about it).
So, to still have Pad1 with Bass and Pad2 with Snare, Pad3 with Hi-hat and so forth (because that feels natural), implementing a GM compatible PAD-to-MIDI Note will need a non-linear approach to mapping those, where (for instance) Pad1 to Pad4 won't correspond to 4 sequential semi-tones: C1 to D#1
The only situation I would see this as being a nightmare, would be when someone would try to control through the Keyboard a perfectly Chromatically mapped sound on the PADs. "Synth Bass" project is a perfect example of that situation.