Pablo is right. 90+ degrees with 80% humidity is far worse than the heat training that anyone needs to do. ALL Florida runners are heat trained, all the time. Doing every single thing you can possibly to do stay cool while running in Florida summers, you are still heat training. It's just that bad sometimes. Florida summers are brutal for running. Actually, from May-late October is brutal. Trust me, they are getting all the heat training they need right now.
Also, the "africans" wearing shirts are most likely training at altitude where the air is dry. There's a HUGE difference between 20% humidity and 80%, or even 60%. I just walked across the street to the cafeteria and my clothes are sticking to me. The humidity here in FL is nasty...and I'm heat acclimated. I used to live in Denver. It gets hot there during the summer. In fact there were 31 days of over 100 degrees in the summer of 2008 there. It was hot, no doubt, but it was nowhere near as uncomforatble as 90 degrees is in on an average florida afternoon.
FL coach - Go to pubmed.com and type in "heat stress running clothing". You will get a number of articles (well...abstracts of articles) that discuss types of clothing for heat stress. I have also contacted several people who are experts in the field asking about this since I cannot personally find much regarding shirtless vs. clothed. Waiting to hear back.