There is nothing wrong with your original plan whatsoever. It's the TB bandwagon getting in your ear.So if i understand correctly, it would be better to use another 1x15'' on top of my own or buy two 2x10 cabinet or whatever i like from the same company?
If i do so, how many wattage should i look at considering my power amp? Since my power amp can take 8 (310 watts), 4 (450 watts) and 2 ohms (800 watts) in stereo, would it be more clever to buy 2 x 4 ohms enclosures instead of a 8 ohms?
BTW Chef, why are you bypassing the biamp option? Does it sound so terribly bad?
Your biamp plan gets you the best possible fidelty in your mids with the best dispersion around the room. The only downside is less volume than you would get from two matching full range cabs. If the old 15 is loud enough already you are good to go!
I've done this with my 15 and 2x10 vertical. It sounds extra good biamped but not worth it for me to carry all the extra gear. I have to EQ for the 2x10 and mind the volume but that's fine for me.
Your power amp isn't the greatest, to be kind to it. I wouldn't invest in 4 ohm cabs in case you wind up in a different amp direction. Whatever power it makes at 8 ohm is plenty for one cab.
2x identical 15's is a nice rig but doesn't have the room dispersion of vertical 2x10's.

