Hi Chris,
The term 'rounded pistons' refer to something we've been doing to the standard pistons for a while now.
If you look closely at a standard piston, the edge where the sidewall meets the upper and lower surfaces is square - i.e there is no radius.
What we've been doing is to introduce a radius on BOTH the upper and lower surfaces to round them in this zone. This has been done ever so technically with an old shock shaft, a dremel and a piece of 600grit wet and dry paper.
From our testing, we believe this makes the shock better at absorbing all the little bumps and ruts that characterise many of our UK style circuits. The down side is that it will rob you of some large jump landing prowess.
Essentially, it's the same as thinking about the old AE square edge pistons on their B4 and the old round edge pistons on the Losi shocks. The Losi ones were always great at small bump performance whilst the AE ones had better big jump landing performance.
What we've done is to introduce a radius to improve the high frequency, small amplitude reaction to make them a bit faster reacting - a bit like taper pistons but the compression and rebound rates stay the same.
In the early days, we were using Nick Gurnell's machined and tapered pistons and the quality of his work is awesome - but we made a choice to stick with the stock kit pistons and to try to make them work so that we could help TLR22 owners who maybe didn't have the option to try's Nick's offerings.
The 55 size hole will give you more pack than the 54s - the 54's can slap through a jump landing and cause the car to double bounce or give an uncontrolled landing. As Chris has said, playing with pistons is pretty much what we do to alter the car's characteristics from one track to another now that we're happy with our own individual set ups
As for the 3 deg Caster, I think you're spot on - the 4mm trailing axle will offset some of the initial aggression from the caster block. As I've said myself, I've not yet found them to be to my liking but there's little denying that Chris has found a set up that's given him a lot more pace recently and he's now regularly too far in front of me.
Ref the +0.75mm hexes - yup, they can be used and still have the car within the BRCA box. Chris ran them pretty much throughout the national season with Schumacher and Ballistic tyres and his car fitted into the BRCA box without any problems - with 4 deg rear toe.
HTH