I'm curious to know which is a better option. I use a shim on my RDM's but find my boom is not as "stiff" as when using it with a SDM. Is it best to use a shim or a RDM boom (mast to boom connection). Cheers.

I like those rubber shims. They provide a bit of give for shock absorption. They don't interfere with the kind of stiffness a boom needs to maintain sail tension. You need bugger all clamping pressure to stop your boom slipping. The only thing wrong with them is they make adjusting the boom height a bit more difficult because they don't move.
Imho they are an improvement on the normal direct connection.

I was speaking to a guy at corros and he reckons that the reason skinnies don't break as often as fatties, is because of that rubber shim.

"Where do SDM masts usually break?" he asked me.
"Just above the boom" I said.
He gave me a knowing wink

I have an rdm mast and also a Chinook boom that fits rdms and sdms without the shim. However I use the shim as I think it gives a small amount of shock absorbtion - have already absolutely wrecked one Chinook boom but with no damage at all to the mast. The shim doesn't make the boom loose (depends on tightness still) just a slight amount of cushioned movement (hardly noticeable).

i have a couple of chinook mast guards i use on my sdm. got them years ago and always use them. the theory was that they spread the load and prevent masts from breaking. has worked so far.

i looked on the chinook site and they don't seem to sell them anymore. the same thing can be made out of pvc pipe with a split in it which is what some of the racers used to use in the 90's.

Thanks all for the advice! I'll stick with the shim's, sounds like they are well worth using.