According to last years river ticket, the speed limit is 6mph up, 8mph down. You can get up to about a mile north of Stourport. Then from Ironbridge to Shrewsbury where there is a derestricted area for watersports.
Be advised that you will be scrutinised through high powered binoculars every inch of the way. Any boat with an engine of more than 10hp will be viewed with the deepest suspicion by an army of ditchcrawlers, anglers, birdwatchers, ramblers and other such sub species. The world speed record for the leg between Upton marina and Worcester bridge stands at 18 minutes and is held by a rib owner who cannot be named for legal reasons. Also for your information, the river police boat can only manage 22knots and usually runs out of petrol three miles short of Worcester.