As far as I understand all 310s were 75 mph only whereas 312s came in two sub classes. The London midland were 75 mph and Great Eastern 90 mph.
Are you sure about the 304s ability to accelerate ? They had 828 hp and i remember like a 305 4 car set that went to Bishops Stortford they were incredibly slow to accelerate. I doubt they bothered the 310/312 units.
In later years the 304s had the trailer lavatory composite removed, so the power-to-weight got a lot better as a 3 car unit.
Apologies, I do believe that the 310s were 75mph, (so maybe the 304s weren't posh enough for the WCML south

). All the 312s were 90mph though.
Even in the 1960s, using 304s on services such as Northampton-Euston when they had no internal gangways and only one car out of 4 had lavatory access wasn't considered adequate. Especially with the number of LMR HQ staff who used the route...
The 312/2s are showing as 75 mph geared units. Maybe it's to retain comparability with 310s. In any case 310s were easily able to exceed 75 even uphill. I remember speeds well above 80 on the speedo going towards Watford Jct from Euston.
.
I've always understood that all the 312s were actually identical but the LM allocated quartet were stencilled for 75mph for safety reasons as they could and did multiple with 304 and 310 units. You really didn't want to be in a 304 unit at speed on the WCML after the 86s had hammered the trackbed, so imagine what it would be like if the unit in front was driven at 90mph.
I suspect the 310s were designed for 90mph, they had B4-derived bogies, but again as long as there were 304s about the marked limit would have to be kept down to 75mph. I spent years commuting on them between the Milton Keynes area and Euston and if they got a clear run on the fast roads they certainly ran well above 75mph. At this period the linespeed on the slows was 75mph, another reason for the official top speed.
We have a winner. The 312s were indeed a modern version of the 310. the big difference was that they gangwayed throughout whereas the 310s were not gangwayed between coaches two and three.
Correct for the units as built. 310s were later rebuilt to be gangwayed throughout.