Voice Apps is the Voicemail software. Nothing exclusive there. Telephony is merely the data within the MSC card. As long as the core loads are the same, there should be no issue restoring from one to the other.
Registry, keycode etc from a FULL backup will cause problems with zero doubt.
I had asked this very question of ITAS some time ago, and we came to the conclusion that as long as you're only restoring those two items mentioned above, you will be fine. The caveats are:
1. Both machines must have identical patch levels
2. Both machines must have identical core loads (including type -- ettiquete vs CT2Plus)
3. ONLY restore telephony and voice apps.
If in doubt about the voice apps, it would only take an hour or so to survey the 200's VM system top to bottom and export all the greetings/ccr prompts. Then rebuild the VM on the 400, import the prompts and the only loss is messages.