The main difference is that for a normal 1400 Sea King engine, any excursions to max contingency (2.5 minutes) are not cumulative and are reset as soon as you lower the lever. The 1400T engine in the RN SK 4s is knackered once you amass a total of 2.5 mins at max con.
It is a short term gain to allow the aircraft to have some useable perfromance in conditions it wasn't designed for.
And soon they are going to fit Carson blades and send it to Afghanistan!!!!