Effects of Brake Master Cylinder Bore Size and Bent Brake Discs:

In general, larger bore sized master cylinders require less brake lever travel for the same amount of brake fluid to be transferred to the caliper and hence move the pistons/brake pads. Many mistaken this as more brake power, but it really didn't give you more braking power but shorter brake lever travel for the same braking power compared to a smaller bore sized master cylinders. Of course, you could probably lock up the wheel easier with the larger bore sized master cylinders since you only need to move the brake lever just a little extra for that extra brake power.

With larger bore sized master cylinders, the brake lever pressure build-up is faster and stronger due to the fact that they move more brake fluid volume with the same brake lever travel. This will make modulation by feel of pressure (pressure build-up/difference) easier as the some smaller bore sized master cylinders didn't have noticeable pressure build-up/difference (the bore is too small in this case). Going too large a bore size will make modulation difficult though as the brake pressure build up is too fast and there isn't much brake lever travel from zero braking to 100% braking to modulate with (more like a "on and off" switch). The smaller bored master cylinders but not too small to lose any noticeable brake pressure build-up nor cause insufficient brake travel within brake lever-to-handlebar design limit to achieve the 100% braking power will make modulation by brake lever travel easier. Some prefer modulating brake power by feel of pressure, and some prefer modulating brake power by brake lever travel. You'll need to find out what modulation style works the best for you (by feel of pressure or amount of travel), then pick the correct bore size/ratio for the best balanced brake pressure build-up and brake lever travel for yourself.

Another advantage of larger bore sized master cylinders is the fact that they move the pistons/brake pads more with the same brake lever travel when compared to the smaller bore sized master cylinders. This is useful when your brake discs are slightly warped/bent (within 1 mm from side to side) which will push the brake pads/pistons out farther when not using the brake as the wheel/brake discs rotate. The slightly warped/bent brake discs will cause you to think that the brake becomes slightly mushy all the time when the bike is moving without the use of brake and required you to pump the brake lever farther or sometime more than one pump depends on your master cylinder bore size to get the brake pressure level back up (move the pistons/brake pads in enough to make solid contact with the brake discs) to get the desired braking power. This is because the pistons/brake pads have to travel more before they make contact with the brake discs again after being pushed out by the warped/bent discs. With the larger bore sized master cylinders, you could save yourself the trouble/panic with just one pump of the brake lever but longer brake lever travel to re-gain the brake pressure to achieve the desired brake power. Of course, this is assuming that you have bled your brake fluid correctly and there is no air bubbles in the system. If the above situation had happened to you (brake felt mushy as soon as the bike is moving although it felt ok after you pumped it a few times while standing still), I would suggest you to check your brake discs for warping/bending first before you bleed the system hundreds of time and still wonder what's wrong with it. Slightly warped/bent brake discs may not be easy to identify with your eyes, and may required some tools (same ones as you used for checking the wheel straightness). If your brake discs are just slightly warped/bent (within 1 mm from side to side), you may be okay to continue to use them if your master cylinder bore size is large enough to enable you to re-gain the brake pressure and desired brake power quickly with one pump. Of course, replacing the warped/bent discs is the only way to cure the REAL problem (mushy brake caused by bent brake discs).

Note that full-floating brake discs may also cause the brake pads/pistons to be pushed back slightly just like slightly warped/bent brake discs. This is especially true when you had the headshake/tank-slapper coming off the exit under hard acceleration. This is because vibration from the violent/more forceful shake could cause the full-floating brake discs to move/float from side to side and hence push against the brake pads/pistons. However, this type of brake mushiness shouldn't be too severe to cause a panic at the end of straight.

If you had to pump the brake several times to get the brake pressure feel back every time after the bike moves, this is a sign that your brake discs are severely warped/bent (more than 2 mm from side to side) and will need replacing ASAP to keep you safe. You should be about to see the noticeable warping/bending easily by looking at the brake discs as the wheel rotates in this case.