Raising the head via head shiums lowers the compression ratio slightly reducing the tendancy to blow glow plugs due to the higher temperature the additional nitro content will cause; this is especially true on a piped engines.
In my case, running my ST X-61 with tuned pipe, on 3 engine runs I blew 2 plugs; not having head shims available, I placed a second plug washer under the glow plug achieveing the same thing as raising the head. I now have several runs on my ST X-61 and no blown plug yet.
So it seems to work. The only difference I noticed, the RPM's on the ground reduced by 165 RPM's, not enough to make a difference.

Orlando