Lol there Isn't other clothes that gives bonus exp besides cat in the hat. If I believe.
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pretty sure there isn't a single item left that does this, unless it's one of the newer added pieces (which I doubt) If there is however one left you'd be better off just telling this to a developer since it's not intended to do that (and who knows, it might stay around for pretty long, the lvl 40 MG with no level requirement set took ages to be fixed to lvl 40 requirement again).
Only the event XP ring and cat in the hat give bonus XP. No other item do so.