I can tell you first hand that the HF and Si trannies, both 5 speeds, are completely different. In fact, for 88-91, there are 3 different 5 speeds and a 4 speed transmission too.
The 4 speed was found in the STD body style or the 70 HP variant of the engine.
The DX 5 speed was found mated to the 92 HP engine. This had 1-5th gears same as the Si but with a taller final drive.
The Si 5speed was with the 108HP engine and had performance FD, ie, slightly smaller. Probably do about 3500-3800 rpm at 80 MPH in 5th.
Now the HF actually had 2 different versions of 5 speeds. They both had taller
1-5th gears. One has a huge FD, something like a 2.82 and the other had a gigantnormous 2.18 FD.
If you had the HP, you could go about 187 MPH [no joke] in 5th gear running about 6500 rpms... That is why the HF gets about 55 MPG on the highway and 49 MPG city.