I would like to add another reference to Medicine and Science in Sports and Exercise (Bloomfield et al). If you are truly wanting to build power, 80% is way too much to be lifting. You are building strength by doing this, which you do need as a base for power. Power is thought to be built optimally by doing sets with 30% max but doing them fast (e.g. jumping squats). Plyometrics will also help, although i assume that you already do that kind of thing in your sport.

On the hamstring note, (and this is hard to explain so bear with me), eccentric exercises would be beneficial. In normal gait, the most strain on the hamstrings is just before heelstrike when the hamstrings must eccentrically contract to slow the limb down. (Eccentric contraction being one where the muscle lengthens under tension instead of shortening). In other words, the hamstrings have to clamp down the brakes in order to stop the momentum hyperextending your knee. As you can imagine, this also happens with kicking. Therefore, doing eccentric exercises is the best way to prepare for this kind of activity.

Specifically, a good (intense) eccentric exercise for the hamstrings is a roman chair type exercise. Kneeling on the ground, you hook your heels under something, then keeping your body in a straight line, try to move closer to the ground (i.e. so you fall flat on your face). It is quite a hard exercise and i wouldn't do more than ten in a row, especially starting off (one set) even if you feel ok then as eccentric exercises tend to make you very sore the next day or so. This is thought to be
because the muscle fibres are being forced to lengthen past their comfort zone whilst still contracting and this breaks off some parts of the fibre. The good thing about it after you have recovered is that this actually lengthens the muscle fibre(rather than increasing size) which helps to prevent injuries at extremes of joint range.