The Drake R15 56 cm would be a major improvement over the stock Drake 41 cm Shallow Water "Cleaver" fin.
The 56 cm will help you to plane earlier, will take you upwind better, so it's a very positive upgrade.
IMHO you do need a larger sail to get the most from your Rio M.
At 90 Kg. you can most likely lower your minimum threshold on the Rio M from the 15+ knots needed with your 6.7 m2 NP Tempo to around 10-12 knots with a 7.5- 8.5 m2 rig.
I checked the NP site on the Tempo and it looks pretty good.
The NP Helium 7.5 or 8.5 on a 100% carbon mast will only weigh about 1 kg. more and give you alot more sailing (planing actually) days on the water.
It might weigh less if you do not have a high carbon 430 mast in your Tempo.
As far as a larger sail taking up more of the deck area on your Rio M, we need to talk about that a bit.
Somehow, you are keeping the sail (or your body) over the board too much.
As the sail is sheeted in and then raked back your body needs to move back on the board and lean outboard to counterbalance the side leeward pull from the sail.
So, your body and the sail should not be vying for the same deck space on the board.
My guess is that you are not moving back far enough (but you soon will) and that you are most likely
oversheeting to some degree.
Keep working on getting planing more, moving back on your board progressively as your speed increases,
and use the larger fin.
Then look for a larger sail and you will be planing far more and your quality time on the water will increase.
Hope this helps,