I assume you are testing with compressed data. Force GT is sandforce thus it won't do as well with incompressible data such as random data.
When I compare it to anandtech data of ASSD benches it looks to be right.
Write perfromance is 160/170 on the 120GB version. Half the space usually means also half the writing speed.
Read speed sounds reasonable too.
What many people don't seem to understand is that for good performance 120GB is the minimum. The smaller the process the less nand chips and the higher you need to go with capacity if you want to get all the benefits of an 8-10 channel interleaved controller.
256GB still performs much better than 120GB. Going up to 512 is not really necessary. 60GB still offers good enough read speeds but for writing you'd be better off with an HDD at least sequentially.
120GB doubles write performance and gets you full speed reads 256GB would increase write performance again but not as much as 120GB already populates all channels afaik, 256GB just interleaves request and can achieve some more performance through that.
Sandforce drive manufactures cheat with the specifications as they can reach those top speeds but only with very well compressible data. Fortunately much system data is compressible for a system drive it isn't that bad of a choice.
HellDriveUK doesn't seem to know what he is talking about. HDDs usually are qually fast in writing and reading and do so sequentially upto 110MB/s current gen 2.5". And normal SSD aren't that slow either.
PS: Compare drive speeds here.
http://www.anandtech.com/bench/Product/402?vs=371
There is only the Force GT 120GB in the database but you can just compare the Agility 3 which is there in many sizes to see what size difference does to performance in each benchmarks.
