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It will only improve your FE if you drive it within the engineered parameters. It's a simple job for anyone who's remotely mechanically inclined, but I'm sure you can find an HX in your area for a reasonable price, they aren't exactly flying off the shelves.
HX also requires special spark plugs and oxygen sensors, not something you have to replace everyday but when you do they are pretty costly items.
The cat is part of the exhaust manifold assembly like on the CX if I remember right.
Gearing probably isn't ideal, but work with what you have, I guess if you are that determined.
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