I have never made the exact measurements that you have made. It sounds like a big differential to me. This it what I would do. Turn laser off. Turn the chiller off. Disconnect the two cooling lines from the backside of the turbo. I believe one is black, the other a yellow color. The two clear ones go to the vacuum pump. THIS IS A MESS. THERE WILL BE WATER EVERYWHERE. ELECTRICAL EQUIPMENT AND WATER, NOT GOOD. Please clean everything up before you turn the machine back on.
Stick a compressed air nozzle in one side and put a rag over the other. Once the water stops, if there is any, then check to see if there is good flow through the turbo's cooling chamber. Also check the rag to see if there are any chunks of mineral build up.
I seem to remember checking the bottom of the turbo's temp before. Its that 6mm black anodized plate. I think it was at 28C but not exactly sure.
When the turbo is failing or has failed, you drain the turbo oil, the L / H sight glass oil level indicator will have particles, grit, black chunks, on it. This is not 100% but a good indicator.
A C1500B has a C009 Turbo in it.