The Futura 141 should be a little easier for you to get accustomed to (coming off the Rio M) as it's
wider and has a little more volume.
It will definitely feel more like a short board than your Rio M ever will.
I think the Rio M is perhaps the best longboard/shortboard "compromise" ever designed, but for the true
shortboard feel and performance, you will do nicely with the Futura 141.
Actually, your 5.0 and 6.0 rigs (particularly the 5.0) are going to be a little small for the F 141 for a guy your size.
This is due to the width and volume of the F 141.
When you get conditions that require a 5.0 m2 rig for a 200 lb. sailor the chop will probably be big enough
that you will wish you were on a 110-122 liter Futura.
But, you have the sails, and when conditions are right for the smaller rigs, I would suggest downsizing the
fin about 4-8 cm and give it a try.
You will be able to sail OK, but the ride may get a little rough. If so, try moving the mast foot back to near the back of the slot to get more of the board out of the water and the center of effort in your small rigs back over the fin's center of lateral resistance.
The 2010 Futura 141 is supposed to come with a 50 cm fin, so look for a 46 and maybe a 42 for your smaller rigs.
Be sure to take some 320-400 grit wet/dry abrasive paper and "fair in" the paint/decals on the OEM stock
fin. These fins are not bad, but often a little "smoothing" of the foil profile can go along ways toward making what seems like a poor fin (spinout prone) into a pretty good fin.
It's not going to help much with uphauling in that much wind, but it may "push" you a bit to work on waterstarts.
Just be careful in < 6.0 m2 conditions as you may find yourself out in conditions that make uphauling nearly impossible.
You will enjoy the Futura 141, I'm pretty sure, just give yourself enough time on the new board to learn
how it works differently than you Rio M, and you should be on your way to full on shortboarding.
Hope this helps,