Yes, due to the exponential-increase in drag and power required to overcome that drag, the actual amount of power generated with furious pedaling doesn't give you much more speed on downhills above 30mph. For example, to go from 20mph to 40mph, you need to generate 8x more power; simply impossible to do if you're already giving it your all at 20mph.

The pull of gravity is a fixed amount as well, but is based upon your total weight. So what I sometime do is wear an empty backpack up a local hillclimb. Then at the top, I fill it with big rocks to add an additional 20-30 lbs to my weight for faster downhills. This gives gravity a stronger pull for the same amount of aero-drag. The more weight you add, the closer your downhill acceleration approaches the theoretical free-fall speed in a vacuum (drop a rock and feather at the same time from the same height, then compare to doing it in a vacuum).

So your best bet to increasing speed is to reduce aero-drag. Tuck in tightly with your knees and elbows in. Head as low as possible with straight flat. If you have the skill and practice, you can even place your hands together just on either side of the stem:



BTW - Craiglist, thrift-stores and garage-sales are great places to pick up used bikes at incredible deals. Due to the "newest is best" fads, there's tremendous depreciation. I've seen carbon-fibre race-bikes with Ultegra components go for $150-200. With Dura-ace at $200-300. A very nice basic steel/alloy bike with low to mid-range components can be had for around $100-150. I picked up a nice alloy MTB with suspension-fork and DeoreXT parts at a local police-auction for $40.