You need to separate the Alumina Clip parts of the bar from the base bar in this discussion. From what I have seen and heard, the alumina clip hardware is very good, adjustable and solid. The aerodynamic differences between those parts and an integrated bar will be minimal. However, the Alumina base bar does not seem to be the most aero base bar out there. A bar like Zipp's Vuka Bull or one of the nicer Profile Design base bars could help you.

And on the issue of the handgrips, you probably could cut them to make the reach more comfortable, and I have been told that the difference between straight hand grips and ones that curve up is basically nil.

So to recap, the Alumina system is probably the best clip on aerobar hardware, although the base bar is not as aero as some more expensive competitors, although the change would in all likelihood have minimal gains.

Also take a look at this Slowtwitch article on selecting a base bar: Article