I agree with most of this, except there are a few observations that migh affect the debate. To start with the 75 mm calibre was the standard tank armament adopted by the Allied forces, and this was because the majority of tanks being fielded into the front line formations were US Shermans. By standardizing the ammunition supply, the Allies achieved a far better logisitical situation that the Germans could only dream of. Given the logistical difficulties faced by the allies following the breakout, I would think attention to logistics to be a far more important issue than an increase in lethality.
You also mention that there was some kind of disaster facing the Allies in their tank formations. Whilst I am the first to acknowledge the tough fight faced by the allies in the Hedgerows, I hardly think it was a disaster.....Allied Tank losses were heavy, but never cataastrophic (unlike the germans experiences). There is much debate in this and other places about the ratio of losses for specific battles, but overall the loss exchange rate was actually heavily in favour of the allies, after the debacles of Falaise, Cobra and the Ardennes are taken into account. As an example, during the vaunted Ardennes campaign, Peipers battlegroup suffered the loss of no less than 33 Panthers for no loss to the US forces, in just one engagement. I am unsure yet as to whether the Panthers were immobilised for lack of fuel (it seems likley), but so what..... a win is a win in my book, and just because the germans chose to adopt a plethora of differnt types, and pursue production choices that sealed their own fate, is still part of the equation in my book.
I actually think the decision to adopt the 75mm calibre was the right decision, whan the whole picutre is considered.