If getting more air out do you need more air in?
 

I put SSI's on my car about a year ago. Huge difference, but what I have found is it runs stronger up to 5000 RPMs then it is seems flatten out. I know that the choke on the air is the exhaust, but with SSI's I have eliminated some of the outging air restriction. Is there a way to get more air in, without adding turbo or a supercharger and keeping the CIS system?

You may need to reset the fuel mixture. Many CIS cars are on the lean side to begin with. Then if you free up the exhaust, it gets worse.

Yes, you need more system throughput to make more power. Your engine is an air pump. It all works together:
1. intake manifold, throttle plate(s), head port design, valve cross sectional area, combustion chamber, cam timing and lift and exhaust manifold and muffler.
2. It seems that Porsche didn't leave a lot of power on the table in any of these given areas.
3. You might pick up 1% to 5% with some changes like you are talking about, but to generate big changes, you really need to made broad changes to the system.
4. prices are effected accordingly.

Doug

Greek55,
The '81 through '83 SC's intake ports are 34mm. This compares to the earlier SC's '78 through '80 which were 39mm. My stock '76 2.7 liter is 35mm.
The SC exhaust ports are all 35mm.
Porsche racing engines designed for high rpm's all ran larger intake ports. As big as 43mm, unless turbo-charged.
The choke point for air flow with your '82 SC is the small intake port size.
If you want high rpm performance you'll need to open up the intake ports.