You already have about the highest airflow you're going to get out of a 15mm fan AFAIK.
Turns out that cfm rating is total nonsense and it's probably cubic meter per hour. Their pressure-flow chart shows 56~ cfm in a no-pressure environment. I looked up a review and at full speed, with a golden motor that hits over 1100RPM it has like 60-70cfm at the most, but mine only hits 1000-1040 RPM so probably around 56-60 cfm.
I can fit a 20mm deep fan too, I just went with the USV 14 because I fell for their rating.
There are a few slightly faster fans but it's not going to make much of a difference.
1200RPM: Thermaltake Luna 14 Slim
1300RPM: Cryorig XT140
If you want more performance you're gonna need a thicker fan.
I have a Cryorig C1 and the performance of the stock XT140 fan was very disappointing. It's loud and doesn't move much air. I replaced it with a Silverstone FHP-141.
If you can fit 120mm fans as well, there's the 20mm thick DeepCool GS120, it goes up to 1800 rpm, so it might move some more air. Not sure if that'll make up for the smaller size tho.
Just wondering, any reason you can't fit a full size (25mm) fan underneath the heatsink?
RAM. The C14S in low profile mode only support 40mm tall RAM with the stock 25mm wide 140mm fan. I have G.Skill Trident Z that's ~44mm tall. Guess there's not really any better options. The USV is already around 60cfm so it's about as good as it gets for now I guess
Could you add another fan for push-pull? How much room do you have on the other side of the C14S?
I can fit another slim fan but I'm starting to think it may not be worth the effort since I tried putting a spare Corsair ML120, just to try it out, and the cooling is still so so compared to the stock 140mm noctua fan. It needs a higher airflow. I think I might try moving to a Noctua U9s, maybe a L12s because I don't see this working out. Another problem I guess is that I just can't supply enough fresh air to the fans because the C14 heatsink is so large it blocks most of the top vent for any intake.
honestly, in your case i think it might be worth it to try removing the heastsinks on your ram, its not easy on the Tridents, but its not terribly hard either.
I certainly considered that. Buying new RAM is an option too since it's pretty cheap nowadays. But the USV slim fans still choked when the side panel was put on compared to open air, so I think the case isn't able to provide enough air anyway. I'm probably going to rule out top down coolers and either go with the u9s or wait for the thermalright silver arrow 130, since the Cerberus has a max cooler height of 148, like absolute max.