Since you removed the thermostat there should be no restrictions on the coolant flow whether cold or warm.
An unsophisticated approach, if you don't care about spilling a LOT of coolant, is to take off the top radiator hose, prop it up, and start the engine. Coolant should FLY out of the top hose if you rev the engine just slightly.
This test is easier than taking off the water pump to check it. If you do this, be gentle with the radiator where the hose connects - its PLASTIC - AHHHHHRRRRRRGGGG!
If you do that you'll want to hose down the car and the area around where coolant spilled.
The other thing that might overheat an engine is a really lean fuel mixture. This is a little consistent with that miss you're experiencing. But I wouldn't think it would overheat around the neighborhood at 25-30 mph.