Before the data is written to the drive it is compressed, so that would not affect the quantity of writes.
Of course it does. If the data is compressed 50% before it is written then potentially only 50% as many sectors need to be written. In the past storage manufacturers always assumed 50% compression ratio for things like tapes, but Sandforce don't make any specific claims. Whatever their lifetime write limit is they are assuming a certain level of compression and thus reduced number of writes.
One interesting thing to note is that encryption will completely break compression on SSDs. Encrypted data is basically uncompressable, so if you use BitLocker, Truecrypt or whatever the Linux/Mac equivalents are then your SSDs lifetime will be reduced. Both Intel and Sandforce claim that their drives encrypt data but are too sketchy on details for it to be of any real use. One would hope that by setting an ATA password it is hashed and used to encrypt the drive key, but neither Intel nor Sandforce have been able to confirm that when asked. Desktop BIOSes almost never let you set an ATA password anyway, and laptop BIOSes usually combine it with the BIOS password which logically must be stored in plaintext somewhere in CMOS memory, making it essentially useless.
I didn't notice any difference when I moved from having Firefox keep cached data on my old HDD to when I got my first SSD and totally disabled any drive caching in Firefox, it only uses RAM to cache data which means that as soon as you close all open Firefox windows all cache is wiped.
This would be an interesting test for you to try. Set Firefox (or whatever your preferred browser is) to "Private Browsing" mode and use it for a while to spot any performance differences. FYI when I stopped using drive caching for Firefox I was on Virgin 10Mb cable, now I am on 30Mb and have not even considered turning caching back on. I did this to reduce the quantity of data being written to my SSD as this was a major concern for potential SSD buyers a couple of years ago.
I find it makes a big difference, but then again I often have 30+ tabs open. I have 16GB of RAM but Firefox doesn't seem to want to use more than about 1GB for memory cache no matter what settings I use. I have Virgin 125Mb but that doesn't make slow web sites any faster. I browse a lot of Japanese sites too and the network latency to the other side of the planet is significant.
Anyway, my point stands, SSDs are IMHO not a simple drop-in replacement for HDDs and particularly on the cheaper models I question the lifetime write limits. My Intel drive had a 3 year warranty and hit the limit after under half that time, so I'm sceptical about Intel's expected durability. My solution is to back up regularly and if it hasn't failed before the end of the warranty I will write to it until it does because otherwise it is going to fail shortly afterwards.