Bad luck Ben, altitude effects people differently and you never know how it's going to hit you until you try it.
i did the Bear 100 a couple of years ago which also in Utah about 100 miles north of SLC - it's similar to Wasatch and spends most time up at 8-11,000' it was my first time at altitude.
I find above 8,000' things get tougher - especially going up hills, and over 10,000' it's very hard! Since then though I've been up at those heights in Tahoe, Transylvania and the Alps and not really noticed it as much. Maybe I'm just a bit more used to it?
if I was going to something like Hardrock I'd get there 1-2 weeks earlier to acclimatise a bit, I have friends who've used those altitude tents, the verdict is out as to whether they helped or not.
As a bit of an appendum to this topic - yes, I did Dragon's Back this year.... and i'm doing asking King Offas Dyke 2016 which is £245. But that's non-stop, no overnight camps...
the only reason I'm doing KODR is I have loads of mates doing it, and the camaraderie will be epic. That's exactly why I did DB which, even at £750 was one of the most rewarding weeks of my life.
when you get into the epic stuff, it's about more than a tent, cup of water and sandwich, (though you get a lot more). The contacts, friends and experience really is priceless - it sounds like a cliche, and probably is. I'm one of the most cynical people around and the £750 I spent doing DB was money o will never regret spending
Good luck MTri, I'm also Cardiff-based. I won't be entering the SW100 but can vouch for the RunWalkCrawl crew having met them, and marshalled at the Vale Ultra earlier in the year. Ben and Joe know their stuff and are veterans of UTLD and Dragon's Back amongst others
You probably already know about the MCN, (Might Contain Nuts) events - ideal prep for the Brecons section and you'll meet a lot of people doing SW100 on those days.
The Beacons are only 45 minutes away, so lots of opportunity to get familiar with the territory - especially the sections in the dark, it should be a cracking course - about time South Wales had a really good 100!