I think there might be some confusion here about the analog world vs. the digital one. When dealing with analog signals, and especially with tape traveling across a head at a certain speed, the RPM of the tape reels makes a massive difference to the potential fidelity of the recording. Faster tape = higher quality (generally speaking).
But in the digital world, there is no need for such concerns. Recording in Reason at a higher tempo and then slowing down the result, isn't going to get you any advantage that I can think of. And surely recording at an extremely low tempo like 15bpm and speeding that up, is definitely going to get all kinds of weird.
Just record at the tempo you think the song should be at, or deal with time stretch/compression & audio quantize to adjust anything as needed.
Exceptions to all the above of course if one is into extreme audio mangling where it might be perfectly sensible to play back something at 5 or 6 bpm and record the result in order to further process it...