Modifications
I was not trying to add to the confusion, nor was I making any recommendations as to what you should do. Merely suggesting you do some homework before making a decision, so as to try and achieve the results you desire.
It can be a rather complicated topic but let me just try and simplify it a little. Most engine modifications will change the characteristics of the output from that engine. The distribution of its output can be spread over a narrow or wide band as well as moved up or down in the RPM range, and the "type" of work it best performs can be altered (often stated as torque and horsepower). This applies to all engine systems and modifications made to them: induction type, cam choice, head work, etc, etc. Furthermore, any/all modifications or changes should 'match' the designed characteristics of all the other systems...they all need to work in harmony.
Since we are talking about exhaust systems here, the design of that system can affect these output characteristics...regardless if it is a cast manifold or tube type 'header'. So for example if you purchase a tube header, find one that will fit your requirements; those of your intended use of the car, those of the rest of the engine's systems, and those of the rest of the car's set up (gear ratios, suspension design, etc). Likewise, if you stick with a cast manifold, select one that will also maximize your particular requirements (as Steve C suggested earlier).
You've stated your intended use is street driving (as opposed to track racing). Also you say your engine must already be kept at higher RPM's to maintain pace, to the point of having to exceed speed limit. So it could likely create a worse situation to make any changes that will move the power band further up the RPM range, or even worse, decrease power in the lower/mid range of the RPM band. However,you could then change the drive train gearing to compensate for this, which will also alter other characteristics. And on it goes.
This is why in my last post I said merely looking at the advertised numbers can be misleading. A product may produce 10 more horsepower, but it may produce it at a RPM you can't effectively take advantage of, and it may actually decrease power at a RPM where you need it most.
Furthermore, don't forget to consider how any changes you make may affect other driving characteristics: fuel mileage, noise levels, vibration, around town (stop and go) drive-ability, maintenance, reliability, resell value...to name a few.
The whole subject is much more involved than what I've stated here. Others may be able to express it in better terms than I have. I recall that Steve C has posted a couple of very involved and very informative threads that give a more technical explanation, perhaps you might want to review those. There are lots of excellent books on the subject as well. But if you are like most of us, you would prefer to have someone just tell you what's the best thing to do. Be aware, another person's recommendations may not match your specific requirements.