Hmm, 54C with that setup, and you are surprised?
That's a 120gph pump going through 3/8" tubing, with an aluminum block and a heater core. I'm surprised water is making it through at all
Seriously though...
First, what is your water temperature? If your water temp is cool, then you can eliminate your radiator and fan as the problem sources.
If the water is cool, then it's the block and/or pump. Exceptionally low flow rates are going to have an impact on heat transfer from the block to the water. And since this is an aluminum block, it will need all the help it can get.
How long is your tubing? Long lengths of 3/8" tubing will add quite a bit of flow restriction. Any tee's or 90 degree elbows? If so, get rid of them - that little pump needs all the help it can get
Sorry to say, but I would not expect that setup to perform any better than a mid-to-high end air cooling setup.