If you've already got the existing (and properly tuned) Celwave 6 cavity notch duplexer, I'd improve the situation by just putting a tuned bandpass cavity on the receive side.
I've always been leery of running notch duplexers on any type of high power/high performance machine. When you're dealing with isolation figures less than 60 dB, every increase in transmit power is also an increase in noise on the receive side. One of my eye opening experiences in the early days of my radio career was watching an old-time tech turn DOWN the power on a repeater to increase the actual performance of the system. Suddenly, handheld portables on the edge of the system were now getting in cleanly - effectively increasing the usable range of the system.
Also, make sure that your jumper cables between the repeater and duplexer are up to the task. I've seen lots of duplexers get blamed for poor performance, when it was really the old, cracked jumpers that were causing all the problems.
I have used some of the 8 cavity notch duplexers for on-site repeaters running at 10 watts or less, and I've found them to be a good option. I've also seen a great variation in the tuning ability of different Chinese suppliers for their duplexers. If you're buying one that is supposed to be tuned, you better have a way to verify it.