They will find that Deane Wood, the owner of Spedeworth, does not take kindly to an organisation acting like a big bully. Since Deane owns the stadium his word is the one that matters.
Spedeworth bought Mildenhall from RDC Promotions when RDC folded due to the legal problems they faced with a newly moved in local who objected to the stadium noise.
I would say that the crowd levels were very similar at most meetings. Both sports had some meetings with better than usual crowds but there was never that much difference.
At one time the Mildenhall fish and chips had a brilliant reputation among the stock car and banger fans.
I found the Rye House chippy was brilliant at speedway meetings but not as good at stock car meetings for some unknown reason.
It was Skegness that had one and a half seasons of Speedway. The stock car track was rebuilt from a narrow tarmac track to a very wide shale track. The track has been changed back to tarmac although it is still wide.
Northampton has the major disadvantage of having a raised centre green which could make things awkward for speedway although Birmingham Wheels had a raised centre green so obviously not a complete barrier to speedway.