Are oversized radiators a good investment for a C4?

Only if cooling is actually the limit. Speedway's view is that bigger isn't automatically better:

Core	Where it fits
1 row	Stock, light duty; the thin parts-store replacement
2 rows	Worthwhile even for stock engines; good for a performance car
3 rows	More capacity for real power; a thicker core
4 rows	Extreme heat: big power, boost with intercoolers, towing, racing

More rows isn't always better: each row adds thickness (measure the space, including fans and shroud — thick cores have hit pulleys and steering boxes), and a thicker core is more restrictive to airflow, so the rear rows do less and the fans work harder. You reach diminishing returns.

Before upsizing, check the shroud, fans, water pump and tune — those are more common causes than radiator size.