I'm using a Fox Float RP3 on my daughter's full suspension bike .
The shock is a spare that I had used for several years on a Giant ATX 990. The current shock has the standard tune from Fox. The valving is not perfect for a faux-bar linkage with a sub 40kg rider.
I recently got hold of another RP3 which was basically rubbish but is a "firm" tune. From disassembly and comparison of these two shocks I've been able to determine what shim changes occur between a standard and firm tune. From this I have been able to extrapolate some valving changes to get a good shock setup for a much lighter rider.
Working on these types of nitrogen pressurised rear shocks is potentially dangerous. Nitrogen fill pressures are between 300 and 500psi depending on the shock. I'm not going to put up any pictures and I'm not going to describe how to disassemble/reassemble the shock. DO NOT muck round with your rear shock unless you absolutely know what you're doing. It's not rocket science but you need to be careful and methodical. If you're both of those things then you will already have done all the internet research you need to know how to service these shocks.
The rebound side of the piston seems to have a few things going on. From the piston face out, there are three large shims (15mm OD x 0.15mm thick, a small clamp, then a type of "check" piston and then a tapering rebound shim stack with a number of clamp shims about 10mm ID, 6mm ID and 0.25mm thick.
Both shocks had 3 of the 15mm OD x 0.15mm shims. I removed one of these to speed up the rebound.
The way the rebound rod sits inside the shaft suggests that the "check" piston and its shims should have some effect on the range of adjustment available at the rebound adjuster knob. Adding shims here will affect the engagement point of the rebound adjuster needle in the adjuster bypass orifice and possibly give you higher rebound control clicks on the adjuster.
Compression is controlled by the belleville washers (sort of conical looking washers). Standard tune has 2, firm tune has 3. In addition, standard tune has an extra spacer shim to ensure that total stack height (with one less belleville washer) is about the same. The belleville washers need to press up against a large thick washer with a bigger diameter hole in the middle to seal the face of the piston.
Propedal engagement point is dependent on pressure on the check valve. The check valve prevents normal flow of oil through the compression circuit. This forces oil though the PP circuit. Rotation of the propedal position knob works a cam that puts less or more pressure on that check valve via a spring. At the top of the spring are some small shims that preload the spring. These are very small (about 3mm OD and 0.25mm thick). I removed two of these (leaving one) to lighten the propedal effect. From my measurements, removing all of them would leave the spring without any preload on the lightest setting. Since the bike has a faux-bar design, it does need some platform to work most efficiently so I chose to leave the last shim.
The other option would have been to change the spring for a different spring rate. However, I've got now way of measuring its spring rate and have no idea if alternate springs are available. The springs in both the normal and firm shocks seemed the same.