Lighten 2 bits means multiplying the channel value by four, making the pixel 4 times more bright. It doesn't sound like an small amount to me.
Also, I don't have the algorithm for contrast enhancement in mind, but 3 seems quite a lot to me.
I do retouch amateur pictures with Gimp. Worse, I usually start with a JPEG, not raw, and I would swear that the output looks far much better than 3 bits per channel (that's 9 bits per pixel). Maybe you are losing some color information, but 15 bits? no way.