M/T Failing? Don't know how to determine . . .

Mazda MX-3 GS SE V6 1.8L
For the most part my Engine seems to be running pretty well. It has a slight sputtering at idle, but I have been through every system I can think of. My timing belt skipped 17 teeth so I did a timing belt/water pump job, and the noisy engine thing is minimal now ('93 MX-3 1.8L V6 with the noisy exhaust cam washer thing). My primary problem is a seemingly HUGE loss of Horsepower. Poor acceleration, down 30-50% I would say. On the expressway, full foot to floor will get me a top of about 81 mph, and, with any slight incline, my speed drops down from that, so, I can barely maintain Highway speeds. Car has 265,000 miles on it, and owes me nothing, but I do need to figure out my main problem. My neighbor suggests a possible failing Manual Transmission. I can see his point; if the M/T is failing, there could be tons of extra friction within it robbing all the lost horsepowers. Thing is this: I have never experienced this kind of failure. Shifting is as good as it has always been for me, owning the car for the last 30k miles. Clutch is fine; nothing is behaving differently about the transmission, other than possible increased internal friction of some sort. Has anyone here had anything like this happen? Also, I have kept close tabs on my cylinder compression. All cylinders remain at + - 200 psi, still, even with so many miles on it. (By the way, the timing belt that came off looked totally OK, but I now understand that we have suspect automatic tensioners. The idlers were slightly worn, but, not enough to skip the T belt). I am currently trying to ratchet up my plans to do a JDM Engine and Transmission Swap because of my big issue. Whatever it might be, it is significant enough to worsen and strand me (again). Any input would be highly welcomed, and thanks in advance.
2013 Mazda CX-5, 2.0 Manual Transmission, 1989 Mazda MX-6
1. I would suspect the timing being off, since it sounds like everything was fine before the timing belt slipped. I have heard of cam being 180 degrees off, but never experienced it since I always had my timing lined up before a timing belt change.
2. If it has a distributor, check the routing to make sure the wires are going to the correct spark plug. Unless that was never touched before you had problems.
3. If it has a distributor, Make sure you have a strong spark. Pickup coil in the distributor can go bad. Also the main coil.