Speed isn't as critical as piling on offense. He gains tons of TM from others landing critical hits (which they are pretty much guaranteed to do after Illuminated Destiny goes off). 4x offense 2x health is what I did. Crit Damage triangle, offense cross, health circle. Then find + % or # offense and speed as secondaries.
Being a tank he won't have exceptional offensive stats but the more you get from mods the more he stacks on his basic and special.
I sliced some decent but not great mods on my Finn and run him at +89 speed, 6000 physical damage, and 78k health. The dmg and health are prior to Number Crunch from R2.
His special hits Nihilus for about 385,000 with 3 stacks of Confuse.