No point remapping a naturally aspirated petrol-there's really very little gain to be found. Diesels run lean and are turbo charged so remapping can increase boost and fuelling.

If you need 'lots of throttle' to maintain 50mph something is very wrong. it seems unlikely on a new Toyota, but has the cat or silencer collapsed? Get an assistant to blip the throttle while you hold your hand over the tail pipe-the exhaust gas should exit fairly briskly. If the exhaust gas wafts out like exhaled cigarette smoke, that might indicate a blockage. Most other faults should trigger EMLs.

This could be a simple fix- take it to a good garage. There is some around.