Some people claim they have no problems with sunsleeves in warmer weather. Of course, there are also some people who say things like riding in an old, white dress shirt is cooler than anything else.

IME, though, sunsleeves are viable as sun blockers (and/or moderate arm warmers) up to about 80F. Above 85F, both the brands I've tried are much hotter than short sleeves and sunscreen. (Both are similar as far as sun blocking and cooling/heating, so I doubt the exact brands are important.) If you've got plenty of water, and not too long a ride, you can squirt water on the sleeves to supplement your sweat; but again in my experience, that only cools me for 5 minutes or so.

Two suggestions: first, try a pair and find out. They're not terribly expensive. Of course, if you can't remember sunscreen, I'm not sure how well you'll remember to take or to put on sunsleeves. Second, figure out how to carry sunscreen with you, and put it on when it's needed. I usually ride with a bar bag, and on longer rides I'll usually stop around 10:00 to put on sunscreen.