anyone know anything about these replacement coolers? are they bigger than standard? greater cooling capacity?
i don't seem to be able to find out much so any help would be appreciated

Yes they are larger, about a third larger than the standard cooler

I have one - don't notice any difference in cooling over standard (although I'm sure there will be) - they are slightly larger than an OEM setup.

The pipe trace isn't as good as standard and you have to clip them up in various places, also, when I had the last oil filter change, the whole sandwich plate came off which wouldn't go back on unless all the pipes were removed from the cooler......

Just go for standard - they last about 8-10 years anyway...

The sandwich plate should be secured with a special large bolt (hollow to allow oil to flow through, IIRC, and threaded at the other end too for the oil filter to screw on to). When I was playing about with options for replacement pipes and coolers, thinkauto came up with a suitable bolt to fit the generic Mocal sandwich plate since the OE item couldn't be used (too large a diameter perhaps, I can't remember why). Presumably the GT3 kit came with a new bolt? Maybe you didn't put it in tight enough?

And although the cooler is wider, if you still use the air duct in the front, the oil cooler will still only "see" the same cross sectional area of air as the OE cooler and therefore not do any extra cooling.

And don't forget the sandwich plate is thermostatic, and controls the flow to the cooler to maintain the temp at 80-90°C IIRC, the only way you'd see any improved performance is by perhaps being better at maintaining the temperature in hot weather and under load.