Here is my opinion...

The type R sway bar is not overkill, it will go very nicely with your setup. You will feel more of a difference with the type r sway bar.

However, if your choices are between the gsr and the Si sway bar, I would get the Si sway bar for the following reasons.

Si: 13mm sway bar. Since you already have the lower control with the necessary hole to attach the sway bar, you will only need the sway bar, endlinks, brackets, 13mm bushing, 13mm bushing holder, 8 bolts and 8 nuts. Price: ~$160 brand new from Honda Automotive Parts

GSR: 14mm sway bar, You will need to get the GSR lower control arm, to attach it(yours won't work, Trust me) you will also need the sway bar, endlinks, brackets, bolts, nuts, bushings, bushings holders. Price: more than the Si setup, cause you will need to get the GSR lower control arms.

If you want a Type R: 22mm sway bar. You may or may not need the GSR lower control arms.(depends on which Type R sway bar you get. There are 2 different kinds), Beaks kit(reinforcement kit so you won't rip the subframe), endlinks, NO brackets, 22mm bushings, bushing holders, bolts and nuts. Price: More than the gsr setup cause you need to buy the Beaks kit. There is a complete kit you can buy at
Beaks Products It has everything you need except for the endlinks.

I also have a set for sale too if you are interested. In fact, I have the entire rear GSR sway bar setup if you want it. It comes COMPLETE with EVERYTHING you need. Price: 150

Hope this helps