I've been making cashes of a number of web pages and compressing them via gzcompress($str, LEVEL OF COMPRESSION).
I've set level of compression to 9, MAX. I know that this requires more computing resources BUT, when I use gzuncompress($str) does it require more CPU usage than if I was uncompressing a string with a lower level of compression?
You should take a string you want to compress and see what difference there is between 1 and 9 levels (I can tell you...not so much). Level 9 compression is just not worth the cpu power, specially if you get a lot of traffic.
P.S. Also I don't see why gzip compress your cached content. PHP can gzip all html content before sending it to client after which client decompresses it himself on the fly.