There is no way you are going to get 80mph out of it without flipping the car over! the car doesn't have any anti-sway bars, it hardly has dampening on the suspension and a wheel base that is about as long as it is wide so at 50mph it's going to get really scary to drive, I've done that going down a really steep hill and I avoid that hill now.
How were you reading the amp draw? going on flat your motor should be drawing about 200 amps, so either you have a bad connection, very clean, snug connections are needed or you will melt battery cables and battery posts! so after driving a short distance feel each connection and when you find one that is warm or hot fix it right away! or if you are only able to draw 300 amps it might be a weak set of batteries or your batteries are not fully charged, if that is the case balance them and fully charge them.
To get a 40-45mph speed you should
look at what TomEV did his goal was that same 40-45mph speed and he spent a lot of time and money to reach that goal, but he did it and he shared how he did it.
What's going to help you maintain that speed going up hills is going to be less weight and a battery pack that can handle high sustained amp draw, lithium batteries are the easy way to get there, 100amp hour or larger cells, a BMS and a charger that is designed for lithium batteries is going to be a bit of an investment but it will also drop the weight of your car by about 400 pounds! $2,500 to $3,500 for a whole set up, but I think it would be worth it.