"Upgrading" for more performance would depend on all the things that Banshee asked about. And, as Doug says ... the factory Manifolds are awful hard to beat on a motor that is stock or close to it. You'd have to go pretty radical on ther cam and heads to need one that's going to flow more.
Is the manifold you have the Offy 360 degree dual port? If so, there is nothing wrong with it - it's just not a high-RPM screaming engine manifold. It has about 6 pounds too much aluminum in it, which is an easy but time consuming thing to take care of. This manifold was conceived from a great idea for balancing the flow through the carburetor and into the ports. Unfortunately, thier casting practices resulted in super-thick webs and in the walls separating the ports. They are restrictive at RPM above 5,000, but below that they are supposed to have really good response and give a little more torque than stock. There are better dual plane manifolds out there - depending on your motor's combination.
I am curious about the EFI. How easy is it to tune and make adjustments?
