You noted you want to keep your power as low as possible. Just curious what the reason is.
I agree that the difference between 10 and 15 watts is negligible (+1.75 dB). Most users (including me) can barely notice a doubling of transmit power (5W to 10W, or +3 dB), even if the user is noisy; they may start to notice a difference if you quadrupled your power (5W to 20W, or +6 dB increase). I always try and improve things by +9-10 dB to make a noticeable difference.
While 9 dB can be attained going from 5W to 40W in one shot, you likely have a good reason for not going there. So get this 9-10 dB from a combination of things - maybe increase power slightly and improve antenna gain slightly.
Maybe go from 5W to 20W (+6 dB), and use a yagi that **has 3 dB greater gain** than your current antenna. That gets you 9 dB total.
Your cable loss is like -1.2 dB (cable plus connector losses). You could halve this to -0.5 dB using better cable (e.g., LMR400 cable), but the cost is high for little gain, considering you are aiming for 9-10 dB overall.
If possible, change antenna height/position to eliminate close-in obstructions (buildings, trees). This can give you 6-9 dB in one shot in some cases.
Finally - if the radio has a "Wideband/Narrowband" setting, make sure you operate "Wideband" or "Wide" if operating through the vast majority of repeaters. This alone can gain you +3 dB of effective improvement, particularly if you are already weak.
Also - if you have a SWR or watt meter, or know somebody who does, make sure the antenna system is performing. Also - make sure you are not trying to use a UHF amateur radio antenna designed for 440-450 MHz, for 462/467 MHz. Some amateur antennas work great on both, but most don't.
Sorry for all the math. We use dB because the values can be easily added and subtracted; easy math for my simple and lazy mind.