I do think 4500w on 240v is way to much for a RIMS regardless of flow rate over the element. At least for my untested RIMS. Its a 1.5" TC tube but the ports are reduced to 1/2" to accommodate my QD's. I do have some 1.5" TC x 3/4" fittings I could switch out to if my flow rate gets bottled up at the fittings.
4500 is only for strike water heating, or mash out if throttled back by a control. I dont have an HLT. I was using my brew kettle to heat strike water. The 4500 will heat strike water at full flow, no problem. It gives about 40f rise to start, then the pid cuts it back as it gets close.