no, you see 50gb used. But internally it is only say 40 gb, saving the flash chips from some dangerous write cycles.
emm, no - the write cycles are not 'ok'. The smaller the structure the less write cycles you get. Modern flash chips write cycles are pathetic.
As one can notice, ext4 does not keep layout close thus requires more seeks to get the data read. Despite that it makes certain operations slower it also will eat your HDD(s) faster.
Check this out - https://lkml.org/lkml/2012/2/29/210
If it's 5 years than even halfing that to 2.5 years is definitely OK. As always, it depends on your usage scenario.
I think Anandtech has something but I can't find it at the moment.
http://www.anandtech.com/show/5508/i...to-sandforce/7 - don't fill a complete Sandforce SSD with a compressed file system, or it's performance (and presumably lifetime) will decrease substantially.