I have no experience of Evans products but here's my take on the subject.

Plus Eight's have always been marginal in terms of temperature control.

The heat transfer into the cockpit footwells can be fearsome on hot days. Coolant will not change this as the lump in front is large, the manifolds bloody hot and the silencers transfer heat into the cockpit under the seats. The corollary to this is that a hot footwell is perfect in winter"

Oil is the primary coolant. Its purpose is to transfer the head created by friction away from the bearing surfaces. A 13 row oil cooler will probably drop the water tempurature by 5 a 10 degrees.

A larger radiator will definitely help. There's three ways of achieving this given the space limitations: a thicker core, a deeper radiator and a thicker, deeper radiator.