Hi, I addressed this a little in the other thread. Many people think bigger is better but when it comes to pumps it is not. You want to be able to turn over the water in a reasonable amount of time, 4-8 hours is what most folks shoot for. Also, slower moving water filters better. Think of it this way, when you wash a car at one of those coin operated car washes, you are using a power washer to clean the car. The force of the water blasts the dirt off the surface of the car. Water in the pool doesn't need to be blasted to be cleaned. It needs to be filtered.
When you filter something, such as when you brew coffee, you are allowing a steady stream of water to seep through a bed of grounds and a barrier that separates the (source) water from the (filtrate) coffee. If you blasted the water through the coffee maker you would end up with watery, nasty coffee containing likely more than just a few grounds. Blasting the water through compromised the barrier, allowing grounds to escape, and the water did not stay in the bed of grounds long enough to absorb enough material to become coffee.
So, if your pump is moving water too fast with too much force, over time it will compromise the barrier (sand latterals, DE grids, or cartridge) allowing dirt to pass through, and the water will not stay in the bed (sand, DE, or cartridge fabric) long enough for the finer dirt to be "caught".
We usually recommend the largest filter you can afford and the smallest pump with enough ooomph to do the job. You could reasonably go for a 3/4hp 2 speed pump and have enough power to run a vacuum. 1 hp 2 speeds are easier to find and should work just as well as the 2 hp he wants to sell you. You just want a clean pool, not a river ride!
To be more technical, look at the gallons per minute (gpm) rate of the pump and the filter. You never want the pump to have a higher gpm than the filter. If you can give us the brand and model of the pump we can help you figure out if what the builder is recommending is a good match.
Edit: You may want to review the info in
this thread. It is much more precise than I could ever hope to be!