OK i also thought about this issue too and this is what i have found out. Block guards are a good thing when installed properly. Let me explain....
In our Honda engines, we have what is known as "open deck blocks" or there is nothing there to keep the cylinders sleeves from "moving" or "vibrating" on the top. The main reason for using a block guard is to create a semi closed deck block which doesn't allow for cylinder vibration. Under high RPM and/or high boost this can happen. In terms of "installing a block guard will allow higher boost levels", i think it's a bunch of crap. Because maximum cylinder pressure capabilities are determined by the cylinder material itself.
Don't bother just having a block guard in a high boost turbo setup. high boost to me is anything over 10 psi. Get it sleeved with a block guard installed by the same person doing the sleeving. A block guard must be installed before cylinder boring and honing.
this will work great on an NA setup too. Once again, you can not just pull the head off, bang in the block guard and go. you should install the block guard, then get the cylinders bored out to the next allowable size on your block. This way if for any reason if your block guard has influenced your cylinders to tilt in any way, your new bore and hone will still be in perfect sync with engine and head. Block guards should be installed about 1/4 inch down from the deck. Then your deck should be resurfaced.
