Remember the quads also drain engine power, thats one reason I didn't keep them on my bug for too long. Also with the Combat impulse engines you get a greater gain for lower power, meaning htat you can have the power low (30 boosted to 60 with skills), and put more power into you Aux, or Shields. I prefer a little more aux so that you turn faster. Although the Bug is the closest you'll ever get to a glass cannon, so shileds are not a bad idea either.
But you won't really need to repec to switch between these two builds. I use the same build for both, (My Defiant, and my Bug)