Thanks for the replies..
I'm using L9i on i7 6700k non delided in sentry case for almost 1 years.. The heat sometimes go to 90 when I'm not using air conditioner..
So for upgrading my L9i, it's better to change it with LP53 + noctua NF-A9x14 (2500rpm) than C7 Cu?
If the C7 Cu is the exact same height as the plain C7, then yes it is probably better to go with the LP53. IIRC, the C7 will perform much better than the nh-l9i if it has enough room above the fan to breath. However in Sentry it has only about 1mm, so it can't efficiently draw in air leading to loud fan noise and performance that is similar to the nh-l9i. AFAIK the LP53 is still one of the best coolers for Sentry.