I did a 10hr pairs XC race last summer, had a nylon Assos jersey and a Rapha Lightweight Merino (Smartwool) jersey. I changed most laps until I found I was getting really clammy and cold in the nylon jersey but stayed nice and snug in the merino top.
I think that is prob is to do with the merino/synthetic contstruction of the Rapha shirt, but I pretty much only wear sportwool jerseys in summer now. Regular merino is lovely as 3 season base layers, but as above it does stay wet and get all heavy, wouldnt wear one in the heat.
Anyone sees any Icebreaker long-sleeve 200 base layers on sale please do a PSA. Lovely fit and last for years.