Did you find your optimun MTU using http://old.testmy.net/t-7808 ?
TCP Optimizer still suggests and loads and saves an MTU of 1500 when my calculated MTU of 1365 gives me the max d/l speeds.
TCP Optimizer gives you the option of loading your own MTU and saving it with it's other calculations. Saving alters your registry slightly.
This change for me 'doubled' my d/l speeds.