Gaining a greater health from Potions?

Went to C2/4371, switched the two bytes to 40 FE

The info funkydisco gave you is right but also false. In a pointer table the offset bytes are inverted. So you'll have to enter FE 40. If you open the disassembly you'll see a "pattern" that the second byte of all entries is between 3E and 42. This sometimes can ring a bell.

Another detail is that the pointer offset he gives is actually special effect $48 but it is true that it is unused. Special effect $00 would be at C2/42E1. I'm too lazy to open usME so you'll have to experiment.

Edit: He probably means special effect $48 is item extra effect $00. I had no knowledge of that.