RE: How to know when you are overpowered?
At 220 lbs. (99.8 Kg.) I think you really weren't as "powered up" as you think on the 6.6 m2 Crossfire. The Crossfire is a very nice sail, but unless you rig if for max. power, it's going to be a little lacking in power compared to some other 6.6 m2 rigs.
The Gaasta Matrix has a bit mor power designed in, and I think your experience showed you that you can certainly handle (at your weight) the larger sail in 18 knots.
Next time you get these conditions, try a couple of things:
First, try to lift slightly (I know this is hard for big guys, but try it anyway) with your front foot and get your board riding more "on the fin". The stock 52 cm should not be too large for someone your size on the GO 155 in 18 knots.
Where are you running your mast foot?
There could be some mast foot tuning that can help your board to go faster and ride more smoothly.
It sounds to me like you are right on the verge of having the all the way back in the straps, sailing on the fin, techniques working for you, but you feel you need a bit more rig to support your weight.
This is pretty normal, and additional TOW (Time on the Water) will get you more comfortable and build the strength in your legs so you don't get leg fatique from pushing across the top of the fin.
Maybe "curl" your foot a little more "around the rail" so you are pushing more on the side of the board, not just across the top under the footstrap.
Also try moving your heel (loosen the footstraps slightly if necessary) along the rail a little forward and aft of straight in the footstrap. You may find a much more comfortable position that gives you even better upwind angle and speed.
I'd suggest looking for a fin that's similar in design to your stock fin, but in about 46-48 cm as this will be a better "step down"" in fin size than the 44 cm freeride fin.
When the wind gets to around 20 knots+, then try your 6.6 m2 and the 44 cm GSport fin.
Hope this helps,