Hi Alex, I ride in outrageously hot weather.
This summer was one of the worse in the last 15 years being an ex-pat in the med.
From Early July up to last week it was constantly 32 Deg at 7am and by 12pm as we were heading back down to the city from the canyons we were riding in 38deg heat.
I have tried everything apart from the wet T-Shirt idea mentioned above.
I only found two things that worked best for me.
Firstly, your technical first layer. What is directly under your suit/jacket. For me, the sweat wicking material that Dainese, AStars etc... sell is by far leaps and bounds better than any normal street clothing items.
It does such a great job at not becoming a sponge that holds all your sweat. You may of course feel hot, but your clothes are not soaking wet. Making me feel way more comfortable on the bike.
I used to ride with a T-Shirt because it was so hot when I first got here from UK, thinking it would suffice but by the end it was completely soaked and made it a disgusting process putting my suit or jacket back on after a sit down.
But that still was not good enough in 35degs on a superbike.
I tried lots of mesh/leather combo jackets, fully vented suits, different brands that have better venting etc...The whole nine yards. But I just cant ride without leathers. I always felt compromised and decided I would rather be hot and safe(er) then slightly less warm and covered in some mesh that may or may not save my skin.
Also, in really hot countries, the problem that I never had in the UK is that the actual air being pushed through your suit is hot! Like really hot! You feel like your in a Fan assisted oven roasting away.
My solution, which I use to this day, together with a sweat wicking undersuit, -if budget permits- and you will need some space under your leathers. Is I went and bought one of those Cool vests that you see the F1 drivers wearing on the grid when its really hot.
I prep it the night before, so its ready.
It lasts around 4 hours. And it considerably makes it feel like 26deg heat instead of 38.
Its costly and maybe someone in the UK may never need it.
But on the continent, over 34deg riding nothing else worked for me.
Enjoy