Oh, I understand the things involving the WAF....but that is exactly why I'd still suggest the spare hd....because the waf is dropping rapidly when the tinkering goes beyond her comprehension ;-) In my opinion it is still easier to set the thing up in one version until its working nicely enough, then switch around an build the other one up. In the end, you wouldn't be able to run two md's and two cores simultaneously on the same infrastructure...so yes, you'd have to build a second network and have all the components twice. Now talking about your amps for example...what good is it to you, if you know that one amp isn't working well with 7.10 and but both work nicely with 8.10.
I've been thinking the same thoughts as you did...in the end, I found out, that what I suggested to you was the best way. 7.10 was running well, but its time is running out. After several trials with 8.10 I made the switch for good. I still have some things to get right, but basically its very safe to assume that with 8.10 you'll get more features and more hardware support than with 7.10, which in the end means less hassle and a rising waf.
Think about it, all you have to do is switch some hd's around and you're done. When you feel like it, you can tinker on the new system, when you feel you need to test out the waf, you just do a quick reboot...sounds pretty nifty to me :-)