Yes, Stonehenge could take more. The restrictions at Knowth and Newgrange are only really there because the parties get to go inside the mounds.

With Stonehenge you could arrive by shuttle from one direction and then leave by a different one, giving you a good fairly long distance view of the monuments from different directions. It would also stop you getting in the way of the view for those approaching while you're leaving. Accompanied by a wider ranging walk around the area would also be a good idea. A longer experience may put a small proportion of people off and keep the numbers a bit lower too.

Knowth & Newgrange cost around €3 each, but it is Ireland's pride and joy and is quite heavily subsidised I believe.