Hold off on the aero stuff until you get all the rest sorted... Seriously. Learn how to "turn AND burn," extracting every last 10th from each corner before you start working on the car any more. With that much power, it's really easy to cover bad driving with sheer acceleration once you get the car straight. Once you really know how to handle a corner (trail-braking, throttle steering, all right at the edge of traction), start analyzing the car's behaviour under different types of corners. increasing/decreasing radius, hairpins, double-apex, chicanes, kinks, carousels, and all variations on- and off-camber, and can blow through them with solid consistency, you really won't know where the shortcomings of the car are. Your comment about front-end stability at high speeds may well be related to spring rate, alignment angles, damper valving, or even the way the REAR suspension is set up... How much anti-squat do you have dialled in out back? Are you lifting the nose under accel, and THAT is making the car feel "greasy?" What tires (brand, model, size, condition and pressure) are you running? What are you static alignment angles?
I think you get my point... I'm sure that I sound like a broken record, but driver first, car last... The better YOU are, the better your car will be, and when there is a weak spot found, you will know with confidence what the issue is, and how to fix it.
Understand that I'm NOT trying to demean your skills, since I have no idea of your track history, but it's solid advice for relatively inexperienced drivers. If you're up to the level where you intentionally use trailing-throttle-oversteer to get the car rotated in the corner, then that's a different matter, and it really comes down to getting the chassis dialled in first, then touching up with aero. Think coilovers with a selection of spring rates, DOT-R tires or slicks, you've done the chassis analysis and optimized all your angles (camber/caster/toe, roll centers front and rear, A/S%, etc.), dialed out bumpsteer, plotted your camber-gain curve, and know how your slip-angle relates to the tractive capability of the tires... My brain hurts just typing this...
For solid info in the area, try
HERE first...