Thanks for your insights everyone. I ended up trading in my i9 MacBook Pro for a Mac Mini as I don't need to travel as part of my new job so a desktop is fine now but I also only have a small desk for a computer. Even if I had the space I wouldn't want to buy another PC with higher base clock speeds just for Secondlife (no matter whether I spend hundreds of hours on SL or not, it's only one application) and for games I have a PS4 Pro, though I rarely get time to use it.
First impressions mirror what others have reported that it performs better than the Intel machine with dedicated graphics that replaced it, but as pointed out that is likely because the base clock speed is higher. @Pauck I haven't heard the fans on the Mac Mini firing up even once since I got it almost a week ago so I wouldn't worry about heat, in fact tests I've seen online show that the MacBook Air only had to throttle down during really taxing and long tests and even then it didn't slow down too aggressively.
I mirrored Pauck's location and settings in Firestorm and at first was getting 18.5fps but it rose to 19.7fps and settled there after I'd rezzed in. I checked Geekbench and the M1 scores a lot higher on single core performance than my old intel chip which may be why it performs better, though multi core was also higher too. And it does all this using so much less wattage, which is quite impressive.
I saw a report today (the video wasn't dated today but it was recent), that the MacMini did at least for a time become the best selling desktop in Japan which is quite an achievement considering the Mac's small PC market share. It's probably not the highest selling in Japan anymore but I've also seen that Microsoft have picked up interest in developing Windows 10 for ARM a lot more as they have ARM powered Surface's so perhaps ARM might actually catch on beyond the Mac line up.
But if LL/Firestorm/whoever else don't make a version of their viewer compiled for Apple silicon then at least emulation isn't proving to be such a huge bottleneck, but we can only see this new systems potential if this happens. Who knows, maybe if they do decide to invest time in developing an ARM client it could be ported over to iOS/iPadOS much more easily (let me dream, even though deep down I know its unlikely :) ). As for the metal/opengl discussion, we'll have to see what happens, perhaps OpenGL on Mac will simply remain in its current form and just not get development (something they haven't developed for a while anyway).
I hate to shill for a company but I trialled a product called Shadow Tech a while ago with Secondlife, there are no doubt other options out there that provide the same service. You basically rent a high spec remote server that you can install applications onto and access it via a client that streams the screen of the machine to you. The performance was great but it came at a monthly subscription cost of course, but for people who don't want a huge power hungry and noisy box in their house it could be a way to go as the client side requirements were minimal, though you need a decent internet connection. You also don't have to worry about upgrades either (or to be less charitable you have no option to upgrade) as its not your machine, they maintain it and you pay them for the privilege. It's not something I want to do but others may find it a compelling option.