My experience in this category is that loosing length produced loss of torque. While I cannot say what the outcome will be with your design, the effect will likely be similar. To me it comes down to what you want. The reasons for shortening must out weigh the compromise in power, and that is a perfectly OK decision if it is what you want. Keep in mind this; the abundance of torque on 919's has many factors besides the pipe. Port size, valve size, cams, throttle body size ECT... Even if you loose a pound or two, or possibly move it up a few hundred RPM, the bike will still retain a good amount of torque. In many cases more than most other inline 4's of that displacement.
In the end, it is your bike, make it what you want it to be and enjoy the results.
Anyway, that is my 2 cents.