Scifi. The speed needed at the top of a loop is dictated by several factors. If too slow then as G0uli correctly stated it becomes harder to keep accurate control of the aircraft, even if the AoA remains very low by unloading. Gravity helps you at the top by providing a 'free' +1g even if you are fully unloaded, but then you'll need more than +1g to finish the loop so you'll need to accelerate to get more speed as you descend. Therefore you clearly can be too slow at the top of a loop.
However, as you eluded to, you can also be too fast. To a certain degree the faster you are the more g you can pull, but the radius also increases so there does become an upper limit. Minimum radius is probably what you are after. Therefore, the apex gate parameters will invariably have a minimum height and a min and max IAS. The Hunter was just above the minimum speed, but not too slow.
The actual speed at the apex will obviously be a function of entry parameters, induced drag and radius. To a certain degree you can pull harder and hence be lower but faster at apex. Alternatively, a slacker pull will give you more height but at the expense of speed. Had the Shoreham Hunter used much less pull on the way up it may have been a little higher but also slower. Entering the loop too slowly also had a significant effect. Of course the change in induced drag with AoA can be considerable in a swept wing machine.
RBaby, I think you know exactly what G0uli was eluding too!