I ran an Ampmaster 4700 on my 280g reef for 9 months, and it did add heat. My tank was around 81F to 83F daily, but I used fans and the window a/c unit to keep the fish room at 77-78F. No chiller.
When I removed it and replaced it with a Little Giant, the tank temperature dropped to 79F, and I've seen the heaters come on during the day, during summer, while the lights were on! And that is without using fans. I still keep the fishroom at 78F during the daytime during summer. That a/c has been off for the past two months due to cooler weather, saving on the electric bill.
I'm sure you'll be happy with your Tunzes. I've got two 6100s in my tank, and they use around 84w. And a VorTech using 26 to 45w depending on my mood. I may get another VorTech and stick it on the back wall. I don't want to add a closed loop. Each of these pumps can move over 3100gph, plus my return pump. Flow isn't an issue, and power consumption is better than running a Sequence Dart pump.