The aircraft 'knows' how much thrust it is getting from the engines and the speed. It should calculate all it needs before even approaching V1 speeds
Posts #445 & 446 in this thread address the idea of assessing takeoff performance early on by use of accelerometers (already onboard in IRS). It's also been discussed elsewhere in PpRuNe. This would cover a multitude of sins - incorrect thrust set thru instrumentation errors, dragging brakes, etc.
It's important to note, though, that ATM (Flex) thrust provides margin built into the system. When you set thrust equivalent to (e.g.) a 48C ambient, the airplane wing is flying in a much cooler air mass; and so the GS (TAS) is lower than if it were 48C outside. So Flex takeoffs have plenty of margin if done correctly.