Taking the dyhedral out of a trainer will make it more agile in the sense that there will not be any forces working against your commands other than the fuse working as a pendulum. So, you will have more control. In terms of flying upside down, taking out the dyhedral will help tremendously but with flat bottom/asymmetrical wings, it is unlikely that you will be too satisfied with the upside-down performance.

In terms of flying faster, it won't. In fact, you can argue that by straightening the wings, you actually increase their efficiency... hence you fly slower. However, I know that many people clip their wings (make it shorter) when they eliminate the dyhedral, which will make the plane fly faster.

Didn't put much effort in refining this post. Hope it is still comprehensible.