I still argue that a trans cooler is a good idea for a modded car. One of my main suspicions is the fact that some Series 1 cars (SSE/SSEi) got them from the factory (external cooler) and these transmissions seem to outlast the much 'younger' 4T65 series.
Many members here, myself included, have had 'impending' doom indicated in our transmissions, and after installing 180° thermostats and tranny coolers, had things settle down.
Additional cooling in the summer months can't hurt your car, but it can extend the lifetime of the fritction-dependent parts and fluid. Overcooling in the winter MIGHT be a problem, but I have yet to read/see any proof of that. If you fear overcooling in the winter, a simple manual bypass valve or thermostat controlled cooler may be your best choice.