I think that it was meant for the Sound storage space to be used mainly as a scratch pad storage... the real "presets" in Tempest are the BEATS, which is probably why the sliders are stored with the BEAT instead of the Sounds... I think it was meant to be used in a way where you quickly dial in a sound to a BEAT, and then tailor that sound with the knobs, and assigning things there... Everything seems to be included in a BEAT... all sounds, all the mixer settings, the distortion, the compressor etc.
Anyway, there are free space for 649 sounds in the sound storage space... this is more than many synths will deliver (todays standard being either 512 or 1024 presets), so I would not say that storage space for sounds are limited compared to normal synths... maybe some users are used to samples where they literally can have tens of thousands of sample files.
Honestly... I would not want to create more than 649 sounds... it would take forever to do that, and too many choices could also get out of hand.
Also, i find that almost all drums loaded need some kind of tweaking anyway, to fit the other sounds in a kit... but I can see why you would not need that, if you're just creating synth banks.
Anyway... this is just my point of view of course