You're worrying over nothing. If it is sitting on its factory-supplied rubber legs, and you're using it for nominal SSB duty cycle, you don't need the fans. If you're using it for near 100% duty cycle (SSTV, RTTY, etc.), then I suggest you buy their fan kit.
Yes, I know it sells for $220 which sound like a lot of cash. However, if you look at its design, you'll see that it properly ducts the air flow over the heat sink. Just blowing air at the heat sink isn't enough!
I own three SG500s. They all have fan kits. The one used on the base station has never gotten hot enough to turn on the fans. In fact, the heat sink barely gets warm.
The ones in the mobile come on occasionally during the hottest of summer days even though they're in the trunk of my Honda Ridgeline. Speaks well for the design I'd say.