It is not supposed to improve IOPS speed directly, just the CPU consumption during I/O primarily which can lead to better IOPS ofcourse if your storage (and implemtation) allows it
I did notice less CPU usage, and better responding proxy. I don't have hard number to share. I never had the opportunity to benchmark it
Ditto, I converted my templates over to the paravirtual controller for both Server and Client VMs about a year ago, and slowly transitioned the VMs from there, never looked back. Zero issues.
Note that while Windows has drivers for it, it won't boot if you just remove the old SCSI controller and add the paravirtual. Need to add the paravirtual separately, start the VM up, let Windows install the driver, then shut down and swap out the controller. Should boot fine at that point.