i did four seasons with that top brace so cant tell you anything about the difference it made... ask howel, he put one on this year so maybe has a better idea if it helps at all.
if you go to one of my early build posts i think you'll find a good list of the the areas i braced. that said, there are often people in the winning circle here that race with totally unbraced frames, so dont get too hung on that. If you are just starting then most certainly it will make no difference to your lap times.
The one thing that worked for me was steepening the rake angle quite a bit by raising the rear by an 1"-1.5". some people here disagree on the need to this, so it's all about personal setup preference.
Good luck!