I don't think there is any need to add more centerboard to the TI, there are many other things you can do to improve the boats performance and pointing ability.
First off it's probably a good idea to never remove the mirage drive (especially the one you are sitting next to), as the boat is a pedal boat first and a sailer second, removing the mirage drive handicaps the boat quite a bit, (especially if running with an empty drive pocket, this create way more drag than the mirage drive, putting the drive plug in helps but I don't think enough to leave the mirage out).
The boat is kind of designed to pedal even when your sailing ( you can pedal very lightly and it still works). By pedaling lightly you increase the performance and pointing ability of the boat considerable (basically what it was designed to do). You will notice much less side slip, and the boat will point higher, and require less rudder (too much rudder steals a lot of your power). Even if you just leave the mirage drives pointed down it has similar benefits if you flat don't want to pedal.
Keep in mind the more you add under water the more drag it creates ( I had a 50 lb weighted center swinging keel (kind of like on monohulls) on my TI for a while mounted in the rear mirage drive socket, and it caused a lot of drag).
Adding a jib also contributes to more side slippage because 1/2 of the power of that jib is lost as side force and heeling force. Unless you go to a wing type jib which doesn't have hardly any side or heeling force at all. I've had jibs on my TI now for over 4 yrs, and find when running the jib I pretty much have to pedal 100% of the time in order to counteract the additional side force (one of the prices you have to pay), which I don't mind doing since this is my exercise program anyway.
Always keep in mind that the TI is not a sailboat, it's a pedal boat/kayak that also sails well, it's not intended to be handled like a laser or sunfish, it has it's own strengths and competing 1=1 with lasers and H16's is not one of them, the TI has a higher purpose ( LOL).
This video shows my TI in 5-7 mph winds sailing mostly up wind (about 15 degress off the wind) so there isn't going to be a lot of side slippage anyway when your pointing that high. But if you look at the boat wake you will see there is almost zero side slip. If I were to stop pedaling the side slip would increase, and eventually the boat would round up into the wind and stall (basically you have to pedal to maintain that artificial trim condition)https://www.youtube.com/watch?v=a1OjgyqBsXk
In the video I think I was traveling around 9-10 mph up wind into a 5-6 mph wind (1.5x windspeed), the apparent wind across the wing (and on my face) was around 15-17mph, the wing in turn directs a very nice organized slipstream across the mainsail forcing the main to work more efficiently. Of course the motors are part of the equation in this situation, but if I were to take the sails down and stop pedaling the motors running at 1/4 throttle would be propelling the boat at around 5mph upwind in these conditions. I much prefer traveling 10-12 mph with a 17 mph wind on my face keeping me cool when it's 90 plus outside ( LOL). Basically the fuel cost for that day was somewhere between $.50 cents and a buck for the day, I gladly pay that to have fun and actually get somewhere (if just sailing only without the wing, motors or mirage drive (on a stock TI), my forward speed would have been 3 mph at best).
I totally cheat and admit it.......