I also built mine with 3" brick, and I'd generally recommend sticking with what has worked well in the past.
I was feeling downright lazy (and cheap) when I built a 42" long oven for a guy who makes the occasional sword. I used cheap no-name (3") IFBs that were much denser than the ones I normally use, and therefore were much poorer insulators: probably significantly worse than 2 1/2" K23s. I used them for the floor and walls without cutting any down. I just routed the element grooves in the walls.
For the roof, I needed to span a 9" gap so used a layer of 1" Ceramic Fiber board with 3" Calcium Silicate board backing it up. I was intending to use 2" Calcium Silicate, but the 3" was free.
It all worked pretty well while the elements lasted. Being for swords, it was only used on Carbon steels and didn't see the sorts of temperatures used for stainless, though it got test-run to 1100 degC (2012 degF) before it went out.
My biggest concern with 2 1/2" bricks would be the skin temperature on the outside wall where the element grooves are on the inside, though I don't actually think it will be a problem. If you can find it, an inch of Calcium Silicate board would probably work well on the outside.
I tend to use a frame built from 1" angle to hold the IFBs together and I also pin them with welding rod if it looks like they need it. I don't skin the outside of my ovens in sheetmetal.
The other HT ovens I've built manage stainless steel temperatures pretty well. 1205 degC is 2201 degF and is pretty much the limit for this 23-inch oven on the 3 kW available from our 13 Amp-fused, 230V domestic power outlets in the UK. It took just about 90 minutes to reach that temperature, though I opened it a couple of times on the way up for photos (the 42-inch sword oven used a separate supply for each of the 2 elements).
Although the surface temperature gets pretty high (I've measured 135 degC, 275 degF with an IR thermometer after a couple of hours running at 1177 degC, 2150 degF), the IFBs are such poor conductors that short-term contact with them does not cause damage to the skin: It works a lot like the silicone protectors clumsy cooks like me put on the front of their oven shelves to save themselves from burns. The Calcium Silicate is much the same as the IFB in this regard.
Putting a highly-conductive metal skin on the outside of an HT oven just seems like a way of making it more dangerous to me, unless there is a decent air gap between the surface of the insulation and the metal skin.
I did a lot of number-crunching when I built my first HT oven and adding insulation to reduce surface temperatures didn't actually stack up very well at all. If you can afford the space to get another inch of insulation all round, the numbers seem to show you are better off going for an air gap instead.
We are on 230V domestic mains here, so 110V was never an issue for me. I'd certainly recommend going for the higher Voltage if you can. Think in terms of safety and use a Residual Current Circuit Breaker (perhaps you'd call it a GFCI over there?) if you do.
