My set up is similar to yours (although my bulb wattage is 75w...and in a 3' viv I'm hitting ideal temperatures at both ends), but I have no heat mat and never have had.
When buying my set-up I took in the 'Leos gain heat through their bellies' arguments, but weighed them up against the fact that, in the wild, the heat would have been provided from overhead and absorbed by the ground. I figured that as I was going to use a substrate other than paper towels, I figured the substrate heat absorption would suffice. And it does. My leos are healthy and have no problems digesting their food. They appear in the evening when the viv temperature dips to it's night-time level, and remain active through the night.
I'm not suggesting my method is any better than the heat-mat option, but as you already have your bulb solution in place I think it's fair to tell you that (in my opinion) further expense is unnecessary.
1.0.0 Bearded Dragon/Killing Machine : Roddy
1.0.0 Crested Gecko : Merlin
0.1.0 Gargoyle Gecko : Amber
1.0.0 Yemen Chamelion : Bruno
0.1.0 Shrieking Cockatiel : Chip