So , I gave it a shot with just MIDI and it was a total fail. I hope others here might know how to do this easier.
I had to match a song once and I used the drums to calculate the tempo. Cubase did a decent job, BUT, I had to do a fair bit of tweaking.
I hope someone else will have an easier way of doing this, but the way I work is that, if I feel I have something I need to lay down , but the click track is killing it, I write freely (one instrument only though). I then switch time base from Musical to Linear (it’s the little clock sign on the channel, it toggles the two).
I then create a tempo track and match the tempo to what I wrote by using the ‘time warp/warp grid’ function. It sounds laborious, but it isn’t really, I have come to work quite fast this way.
But,
The way you describe it, would mean that you would have to switch all channels to Linear Mode and fix the tempo manually to fit the tempo of your song/performance, sorry to not give you better news.
This has been also discussed here: How to make a tempo map of a MIDI track? - Cubase - Steinberg Forums
As you can see, the function is not working well.
If I’m not mistaken, I think, Logic is very good at tempo detection. If you own Logic or know anyone who owns Logic, you could export a track that can feed a tempo to the program, export MIDI from Logic, open MIDI in Cubase on a new session, export the tempo and then import the tempo to your song.
Sorry I couldn’t help any further, perhaps someone else here can give you advice on how to do this easier.