i think the tomcat was slightly heavier masking the weight of the engine, the older 220 gsi was really light without the engine and it wasn't the car itself his car was tidy slightly tuned to high standard and polybushed throughout, straight line it was rapid like you say the weight of the engine will aid traction but when it came to cornering it was a bag of, wasn't slow but could tell the engine was it's let down, he used to get alot of chassi flex put a strut brace in and it cured it a bit but not completely, but the older 220 gsi wasn't designed for that engine it was just thrown in so the chassi was never designed to carry the extra weight, iirc the engine weight about 220kg on it's own "maybe" with the box can't remem exactly it is one heavy lump though, the T series was an old diesel block iirc thats why it was so heavy.

having said that it used to be so funny having people in beemers and stuff looking at you in a "crappy rover" and then just leaving them in a cloud of dust