high water pressure in bathtub but not from the shower head!
Take a gallon bucket and measure how long it takes to fill the bucket from the tub faucet, the shower head, and the shower pipe without the head on it.  If the time to fill is relatively close between filling with the tub faucet and the headless shower, then it is probably the shower head.  The shower head you buy must not allow more than 2.5 gallons per minute in the US, many are below 2.0 GPM.  There are ways around it, such as modifying your shower head or putting in more than one shower head.