Mustangs?  v6 in the base model, v8 in the GT.  Basically the same car on the inside I think.
hyundai genesis coupe.  2.0 turbo up to a 3.8v6
wrx, toyota corolla/martrix, honda civic, camaro, etc etc etc.
the corolla/matrix and civic are probably what you're looking for.  the xrs models of the corolla just had a toyota camry engine in them...and the base civic SI models just have a 2.0l
		
		
	 
Mustang V6 and V8 have different suspensions, brakes, rear end, radiator, exhaust and so on.
I'm sure the Genesis coupe has some difference to handle the extra power.
Camaro V8 has Brembo brakes, different exhaust, suspension, hood and so on.
WRX to STi has a different transmission as well as other tweaks.
The Civic SI is just not a simple pump in displacement.  It takes higher quality internals to withstand the higher RPMS and then the suspension and transmission are different.
Even the Corolla/Matrix example has a different transmission, suspension, and exhaust components.
I simply have not seen a case where a company offers the exact same car with just a bump in displacement for a price.  There are always other components that go with the engine whether it be cosmetic or suspension, brakes and transmission.