Hi Stu,
I'm not sure what IEM system you're using, but I can relate to the in-ear sound being different to the front of house sound, and this does, without a doubt, affect your playing.
To get round this I connect my line-out from the amp to the PA, but this goes thru my own mini-mixer. I connect a spare PA monitor outs to one channel of my mixer, and then use Shure E2 earphones. I had to spend a bit of time at a live gig to get the PA monitor mix right, but once that was done it hardly changes gig to gig. My amps line-out goes to another channel of my mixer, so I can be as loud as I want in the ears. Plus I have 4-band eq on the mixer to play with. It works for me and I can be as loud as I like without the drummer asking me to turn down!
Cheers
Ian