I may have mislead about my element - the 4500W is in my BK and HLT. The one in my RIMS tube is a 5400W element running at 120V - so effectively 1350W.
My cold water test lat night was a "step" from 50-100, 100-130, and 130-145. It seems the water in the mash tun would lag behind the element temp by a minute or so - ie, when element probe showed 98, mash would show 94 (or so). Once the RIMS tube reached 100, the MT reached about a minute later. This seemed fairly consistent on all the temps and I double checked with a thermometer. In water, which is not a good test obviously, I got about 2 degrees rise per minute, depending on the flow rate. It maintained the temperatures perfectly then. I will probably put some pipe insulation on my hoses, just bcs they where pretty warm.
I will probably remount it before I do an actual mash with it - I ordered a few fittings so I could redo the outlet side. I will probably make a couple of batches of mead this weekend since I won't be brewing...