if your team can't catch up, +3 shoes is useless, i prefer Sorcerer's Shoes because you will do so much more dmg from poison and fling.

The principal action from singed is Fling, and whit boots of mobility , you can take him fast, and make a good work for team

Boots of mobility are useless on him. Your trail will make you loose the +5 MS.
Gettin +3 is the best choice, other ones could be merc or sorc ones for def or off.