Hi Thomas,
You may not be getting an accurate picture of your sustained write
throughput using dd.
Since you are using ext4, Iām assuming you are using Linux 
Linux does a good job caching to RAM before writing to disk, so when you
write only 1GB using dd, you are mostly exercising your RAM (assuming
you
have a normal amount of RAM).
If you run a test writing a file much larger than your RAM, you should
see
something closer to your true sustained write speed.
For example, I get the following results running your test, which make
this
caching very obvious:
[ Desktop]$ dd if=/dev/zero of=/home/amadsen/test.bin bs=10K
count=100000
100000+0 records in
100000+0 records out
1024000000 bytes (1.0 GB) copied, 2.62228 s, 391 MB/s
If I increase the amount by a factor of 10, the system is forced to
actually
write to disk finally and the throughput falls:
[Desktop]$ dd if=/dev/zero of=/home/amadsen/test.bin bs=100K
count=100000
100000+0 records in
100000+0 records out
10240000000 bytes (10 GB) copied, 77.4247 s, 132 MB/s
Here is a site I use for checking out drive performance, there is a lot
of
difference between different SSDs.
http://www.tomshardware.com/charts/ssd-charts-2010/Fresh-state-Throughput-Write-Average,2314.html
Good luck!
Aaron