AFAIK, such separation is "legal" in VMC. When I was new to the 747, on a sunny afternoon they vectored me to left downwind/base and then turned me to final on 28L right next to a 777 going to 28R. They called the traffic and we acknowledged it, and there was no speed/overtake restriction. However, after we were established next to him, we got a TCAS RA for him! We decided to continue...
Another time we were coming in on 28R on a long final, and an MD-80 came diving in from the stratosphere to 28L. We were told to maintain visual separation "behind" him. That was well & good until he finally got down to a normal G/S and slowed down. We barely were able to do so in the remaining distance.
I recommend you tell the controller "We're currently at 140 knots, minimum approach speed; unable to slow further to remain behind traffic." Put the onus on him to give you another clearance or make you go around.