Interesting, the reason I mentioned the sticky side seals and Seafoam treatment is I have a similar weak compression issue on my FC that appears to be seasonally tied to my car's usage. I get to a track day maybe once a year if I'm lucky, and my FC gets some spirited use on the twisty roads when the weather is nice but it's basically street driven much of the time from about March - Dec (or whenever the roads aren't salted and the weather is decent). I give it an annual flogging at Deals Gap when I do my annual pilgrimage to Deals Gap Rotary Rally in April/May and rack up around 1700 miles over that week. I have the same RCT5 compression tester you have, and I discovered that if my car is consistently driven frequently & hard, I get better compression figures. And when the car comes out of winter hibernation, my front rotor is marginally weak on one chamber. Eg, my normal "good" readings are ~118's on the rear, and ~108's on the front, all faces. I attribute the "normal" front to rear difference to the fact that I built up the front rotor first, so I probably took off a little too much material when I was setting side seal clearance. The post-hibernation readings are typically 105's rear (all faces), and mid-90's front, but with one chamber in the low 80's. When that happens, I'll add 2 cans of Seafoam to an almost empty tank of gas and take a bunch of full throttle rips to redline in 3rd/4th gear, and then retest the compression, which will magically bounces back to the "good" readings.
I built my FC motor 2 years ago, it's a 13BT, but a bit of a mutt - started with all new parts, S4 housings, and S5 irons & rotating assembly. Put a big street port on it with Pineapple racing intake & exhaust templates. I'm running the S5 OMP with the RA OMP adapter so it's apex seals only see pre-mix, which is gravity fed from a separate tank; engine management (and OMP control) is with an AEM Infinity ECU. Reason I'm on this forum is I just picked up an FD that is running a Link G+ Fury with a single turbo. Trying to climb the learning curve with the Link, which seems much easier that my AEM Infinity! Previous owner chucked the OMP and is running a DBW setup on it - I'm going to have to rewire it all so I can get the OMP back and plan to set it up just like my FC