I can't speak to the newer formula boards (mine is a 160). The same max width (100cm), but the newer boards are wider in the tail.
Reaching on a formula board, as you have discovered, can be quite fun if you have the right size sail. Most formula racers rig for upwind and downwind racing and are tremendously over rigged for reaching.
I don't find my board uncomfortable on a reach unless I get overpowered, and then it's not comfort, it's control. Chop can make the ride a little rough, but it's not too bad.
I stick with my Deb R-13 M 70 cm fin almost all the time and I weigh 77kg. A smaller fin would no doubt provide more control if well powered on a reach, but if it it's early planing you are looking for, you will lose a little bit with a smaller fin. My largest sail is an 11.0.
A newer 100 cm formula board will give you only a very small advantage when it comes to early planing over your 90 cm board (1-2 knots). However, many of the newer boards are much better overall than the old boards (8 years ago).
If the majority of your sailing is in light winds with big sails, it may be worth moving to a newer formula board if it is a good deal. However, some of them were not all that great, so you will want to check it out on the forum before buying. I started with a starboard 175 (not so great), then the 147 (Very good), then the 160 (excellent). I don't race much anymore, but the 160 was and is a great board so as long as it stays in good condition, I will not move to a newer board.
Formula 160; iSonic 111; HiFly Move 105; Tiga 263; '85 Mistral Superlight.
Maui Sails TR 11.0; 9.2; 8.4; 7.6; 6.6; Maui Sails Switch 6.0; 5.2; Maui Sails Global 4.5; 4.0.