Well done - I've never been quite sure what effect lowering the MTU has in terms of download speed, though assumed its hardly noticeable. This shows I wasn't right:
Found on http://www.dslreports.com/faq/faq/2.+DSL+201#7801 Logically, you would also like to get as much data on each transfer as they are willing to send, so you would want select an MTU of 1500 (if your ISP supports it). This would ensure the best possible transfers. If you are using router PPPoE, then your max MTU as allowed by your ISPs and the PPPoE protocol is 1492. Other versions of PPPoE have maximum MTUs of 1400-1492(1438 max for AOL Plus, but 1400 is a better setting for AOL.) You may need to check with your ISP to find out what the maximum MTU is for your network. Setting an MTU that is too small or too great can have extremely deleterious effects on your broadband preformance. Altering your MTU will not affect your latency or TTL. Adjusting the MTU to its ideal setting creates more efficient transfers and thus better overall performance. A well-tweaked system can have high speed, fewer errors and better transfer efficiency.