>> so what is best? the highest figure of 1500 or the lower figure of 1430?
Hard to say - depends on your own system, ISP routing, if its via BTw and your speed.
Try it and see.
All I will maintain is that when on Maxdsl 1430 worked best for me.
Now Im with Be* Im using 1500 MTU .. but you must ensure that you set a high RWIN to get optimum speed.
I spent a fair bit of time tweaking in the wee small hours when my speed was fairly static to be able to notice any increases best. For my own speed (I can sync at 24Mb) I need a high RWIN and therefore I calculated one based around the multiplier that previously worked best for me and adjusted it for the higher speeds.
Im currently using 513920 RWIN. Although I can never be certain, I think I got a speed increase of around 600-700 kbps by changing the RWIN.
You can see my results from dslreports
http://www.dslreports.com/tweakr/block:59168f5?verbose=1&service=dsl&speed=24000&os=winXP&via=routerpppoeAs you can see it looks pretty healthy.