Power to Weight

I've heard that fast vs faster basically comes down to power to weight ratio. By power I mean power to the ground in whatever package is used to do it. It seems logical to me that the more radical you go with an engine the reliability curve goes down somewhat. Not always and maybe not proportionally but some at least. The other side of the coin is removing weight. Free to a point and then probably just as costly as anything else. All else being equal what is weight removal worth in terms of performance. A more abstract question might be what is weight removal worth vs the same cost in reliability. By that I mean if you drop 800 bucks to remove 150 pounds are you better off to do that or leave the weight alone and put the 800 in the engine. These are just off the wall numbers by the way.