Is Thunderbreaker's shark wave useless because of the Blast buff?

You could also just not max booster, then go back and max it with the points you would have placed in LOL shockwave.

And the points that you would put in Speed infusion, since it's the same increase (decrease?) at all levels.

If you have a White Knight/Paladin in your party with Combat Orders, you can get Speed Infusion to 21, meaning a -3 attacking speed. So I would leave SI at at least 19.

I agree with the Shockwave part, although it's stronger then Sharkwave now:

Mobskills:

Skill	% damage	Delay	% per second
Somersault Kick	200%	840 ms	238%/s
Energy Blast	580%	720 ms	806%/s
Shockwave	750%	1230 ms	610%/s
Shark Wave	510%	1050 ms	486%/s

Single Target skills: (comparison)

Skill	% damage	Delay	% per second
Straight	260%	600 ms	433%/s
Energy Drain	400%	600 ms	667%/s
Barrage	2160%	2430 ms	889%/s

Info from here. All numbers ignoring the range, which you shouldn't do when you choose the skills.