I think that the RP23 is an 07 model year shock, and I've heard that Santa Cruz may be discontinuing the Heckler for 2007 (or hopefully just overhauling it like they did in 02), meaning that SC doesn't have any RP23s on order for 06 Heckler frames.
What year is your frame/shock? I have an new Heckler with a Propedal float R, and it seems like a bad match for the frame. Since the RP3/RP23 are also propedal XC shocks, I don't think that they would be that much of an improvement anyway. A DHX air would fit on my bike, but instead of dropping 6 or 7 bones on a new shock, I'm just going to send my Float to PUSH next year for a full upgrade. By the time the rainy season is over it will probably need an overhaul anyway, and I can put up with the small-bump harshness until then. Plus their shock upgrades cost less than 200 bucks plus shipping. Unless you absolutely hate your Float, or plan on doing serious jumps and drops, you might think about doing the same thing.