Exhaust set-ups - what's good & what's not?

I'm interested in trying to make the K 1.8 motor 'breath' easier for the following reasons: improved throttle response, power available lower in the rev range, economy.

I'm not trying to create a race machine - reliability is key.

I'm getting a better picture in my own head of what's worth while and what to leave alone.

The next area I'm researching is exhaust systems. S/S, sport cat Vs no cat, headers / extractors. I know the 'right' size is critical. Just the right amount of 'pneumatic?' resistance to cause that lower pressure zone [delta wave] behind each cylinder's exhaust pulse which helps to suck the next pulse along.

Does anyone have info' on the S/S sports system offered by Rimmer Bro's. part number LF 1048.

I'm not looking for a MG's throaty roar when pulling away.

Is it straighter than the st'd system?

Cat' converter: emission reg's aren't as stringent here, so a sports cat or no cat?
Benefits of sports cat - benefits of no cat any potential negative effects on power/economy?

Extractors / headers: Rover 200 1600 had a 4 2 1 set up, my R25 has a 4 into 1 set up with the cat as part of it. Is the st'd FL the same as the R 25 [1.6] / MG ZR?

HTR

Exhaust manifold (header) design is a long and complex subject. On a Freelander, high torque is more important than ultimate BHP so primary length can be used to improve low down torque. The longer the better is the way to go and not to large either. The key is to keep the gas speed up.
Dumping the cat and swapping the silencers for absorption type will also be a benefit. Keep pipe diameter to 2 1/4" to maintain exhaust velocity. The pipe bends are important, make sure any bends are mandrel formed instead of the more normal crush formed variety.
Avoid fancy tail pipes, a simple exit is much better than some big bore thing or splitting to a twin exit for show.

Much more than that, you are limited to what can be done by the ECU.
For ultimate improvement in economy, you will need an after market ECU. This can then be calibrated to give a suitably lean mixture under cruise conditions instead of keeping the mixture in the lambda window like the factory ECU will.