Terry,
IMO it's not worth paying twice the price of a P7 for an M8.
When I received my first Lower Saxony Police (surplus) import from CDNN I was overwhelmed at how accurate it is, and how much I enjoyed shooting it.
I was impressed enough that I wanted more, (while they were readily available) and my options were to buy 2 more "A" grade P7's, or one HK P7M8. I got on several USA forums (The High Road/Firing Line/Parkcitiestactical), and others seeking input as to which way to go.
The general consensus was that if you are "into" collecting, the M8 would be the best route to take, but if you were "into" pleasure plinking, or carrying the pistol for self defense the P7 was the better choice.
Or, if you are buying the HK to look at and expect to appreciate then no doubt putting your money on a pristine HK P7M8 would be wise.
But I don't "collect" guns, I buy them to shoot and enjoy. The heat shield on the M8's will not stop heat, it just slows it down, or delays it a bit.. The dual grip mounted safeties of the M8 attract some, others prefer the European Heel Release. The triggerguard of the P7 is slightly smaller, and it seems knowledgeable folks who carry the P7 are of the opinion it is slightly easier to carry concealed.
I have found that if you are looking for a point when the HK P7 gets pretty warm then firing 5-6 mags in rapid sequence will do it for you. My buddies and I were shooting my P7 when I first got it, and after about two mags each the pistol started warming up to the point you would think a couple of more mags fired in a row and it would be uncomfortable to shoot.
I know you probably don't do this in Canada (or most can't), but here I use the P7 as my concealed carry weapon some of the time. I don't have an extra mag with me, and I consider a capacity of 8+1 sufficient to resolve any (hopefully none) potential situations. So, a pistol that warms up, or gets blazingly hot after more than 2 mags is of no concern to me.
My choice is not necessarily the choice another person would make. I just think the P7 "looks better", and that's why I bought three of them. No problem with M8's, I just would rather have two "A" grade P7's than one M8.
But, that's just me.
You pays you money, and takes you choice.

Either way you wind up with a fantastic firearm.
Hope you can find one. Oh yeah, try and get all the mags you need when you buy the pistol because sometimes extra mags are a bit hard to find.
Best Wishes,
Jesse