I wouldn't buy one for top level showing. If you want a proper show cob go down the Gwynfaes/Llanarth/Pennal lines. I find Trevallian too horse-like nowadays.
They’re nice horses but I wouldn’t say they’re show type, let alone top level, the stud owner at one time was pretty fixated on palominos and double dilutes and it often ended up with them using not so good stock to get the colour.
I had a Trevallian Ap Comet a few years ago, he was a horse not a pony like Elf says. 16h of pure power and attitude. Jump anything you aimed at, intentionally or not, frightened the bejesus out of me on a regular basis, but what a hunter he was. Soundest horse I have ever had and lived until 28. Superb temperament, very kind and honest but not for the faint hearted under saddle.
As a solid performance horse that happens to be a registered native, then that's what they mostly are.
But not for native showing classes imho, not these days.
AS elf said they are generally bigger more horse like cobs. Nebo, pennal, thorneyside, llanarth, Derwen lines usually produce more top level show types. Any stud that has gone down the dilute route usually has to 'breed in' the colour which often' breeds out' the quality for a few generations.