They should be super slow when walking. If you want to do an ability which mimics their wheel form but negates shields and attack, that's fine. But they were super slow in walking - click, click, click. They were basically tripods when deployed.
But eight movement for two actions would mean they weren't fast even in wheel form, just the same as anyone with speed four.
How about give them speed six, but have a special rule: if you perform an action other than Move, reduce your speed to 1 for the rest of this round.