No cage with a mild steel hoop will cert quicker than 8.50. A 25.5 cert is good to 7.50
That is BS, just check the actual spec manual for both 25.4 and 25.5 and it's clear that mild steel is permitted. I know of several 7.50 cert cars constructed with 100% mild steel, some 25.4 and some 25.5, not recommending it, but it is certainly doable