Mixing Brake Pads On An Axle

What would happen if one mixed brake pad compounds on an axle? I've been wondering about this because a Miata takes forever to wear down brake pads and I have some pads that are not well matched to the car, other pads on the car or I just can't figure out how to make them work. At the last event out of necessity I tried this setup and can comment on its potency. Spoiler: it is not a good thing.

Setup

My normal brake pad setup at the time was Raybestos ST-43 fronts with PFC 08 rears. The 08 rears are a bit too powerful for the ST-43 fronts but the car is drivable.

I found out one morning that likely due to a dead wheel bearing one side of the car wore its front brake pads down to metal. Hence I had two ST-43 fronts and four Carbotech XP10 fronts to choose from. My spare was supposed to be PFC 14s but I could not find them at the time. In the rear I had a set of Carbotech XP8s in addition to the PFC 08s on the car.

Initially I just put XP10s in the front, keeping PFC 08s in the rear, and this combination was way too rear biased. I increased brake zone lengths by 1.5-2x to keep the car composed. To try to fix this, I split my pair of ST-43s to be in the outside front positions with the better pair of XP10s in the inside front positions and in the rear I split PFC 08s with XP8s. I only ran two races on this setup - about an hour of track time.

Front

Looking at the front pads post race, the first observation was that the ST-43s were seriously beat up - chunks of friction material missing, severe taper, a lot of wear for the amount of time the pads were on the car. The left pad was missing more chunks and seemed to be more worn, the right pad was more tapered to the point of being barely usable.

Outer rotor surfaces on both sides of the car were uneven in height radially, showed heat cracks and especially on the left had what could have been pad friction material embedded into the rotor in spots.

XP10 pads on the other hand looked like they did not get worn at all. The right XP10 pad however had a bent backing plate that separated from the friction material - something that happened to me with Hawk pads before. After going out for one session with XP10 in the front and PFC 08 in the rear XP10s became just about white in coolor, suggesting they got extremely hot. Perhaps that caused the backing plate to bend, perhaps subsequent sessions with split pad setup - it's impossible for me to tell.

Regardless, the picture seems to be consistent with ST-43 pads - being the more aggressive of the two - doing double the work on one side of the rotor with the XP10 pads just idling most of the time. The result was a dramatically increased wear rate on the ST-43s as well as increased temperature of the entire braking system. The irony here is that I was trying to get XP10s worn down and not ST-43s...

Rear

I've been running XP8/PFC 08 split in the rear because PFC 08 are too aggressive, and this seemed "fine". Given the front experience I'm starting to think that pad compound split in the rear is not accomplishing a lot. Given lower temperatures of the rear brakes the higher friction pads are probably not wearing down that much quicker, but I am now questioning what kind of braking torque the split rear setup is providing.