Like many on here, I've been at this most of my life. I'm 71.
I work with a LOT better quality test equipment, and have tuned or evaluated hundreds of duplexers on dozens of systems.
Your duplexer in that repeater is probably working as it is supposed to.
Assume for discussion that the transmit radio really puts out 50 watts.
The duplexers of that type of GOOD quality have insertion loss of say 1.2 dB give or take a few tenths.
Add to that connectors from the TX radio, some coax from the radio to the duplexer, then from the duplexer to the back of the
repeater cabinet, and voila, you loose around a dB and a half very often.
So if you are getting 30 watts, you are doing fine.
And in my experience, getting a half a dB in TX output to the antenna is not something that you will be able to notice.
I feel it takes more like 3 dB improvement to be noticeable by casual use.
As others have said, get a better antenna, or much better feedline. DON'T USE ANY ADAPTERS, THEY ARE LOSSY AND TROUBLE.
And keep in mind if you can't make a few dB improvement, then save your money.
A couple dB won't do much except drain your wallet.
Let the flame wars start...