I have a 94 GEN V 7.4L 330, is there any real performance gain putting on EMI thunder manifolds and tube style risers?

In a word, no.

It's all about the total combination of engine componentry. Adding one item alone doesn't make much difference and the emi thunders are only a small step up from a stock manifold.

I'm eventually doing rectangle heads/intake, and cam. I dont want to get too crazy with the stock bottom end, maybe 375-400hp is my target. I can get this exhaust for cheap, I figured a good step forward in direction.

If the exhaust has the dry joint between the manifold and riser, then it is a good upgrade just for the sake of longevity. The stock 496 manifolds also have this, and flow a little better than the regular stock 454 manifolds and risers. You could probably find a set of these as take offs in the $500 range if you look long enough.

You will not gain any performance.

I was in your shoes a couple years back. 95 carbd 454, not a mag engine. I bolted on Dana exhaust, air gap intake, Holley carb and MSD ignition. Ran much better, I'm saying it was from the fresh carb more than anything. I then did rectangular heads and performance took a step backwards until I pulled the engine and did the cam. That's when it all came together. So +1 for doing an entire package, just have the right expectation when throwing a single performance part on.

well I went ahead and bought the exhaust, they are actually IMCO Powerflows, NOT EMI like I thought, but I got them for $500 shipped to my door.

Sounds like a good deal. Have they been pressure tested? Fresh water or salt?

Not bad price as long as fresh water or low,low hour salt.

If anything it should sound a little better.