Corolla S 2003 Naturally Aspired Modifications

Depends on what your power goals are. If you are looking to add 5-10HP (3-8WHP), then you're pretty much on target with those mods. Pretty much all the mods you listed will get you some power, being typical bolt ones, but not the most you could get. To help it along even more is to dump the heavy wheels and tires, as they are not helping you any. Even with the best bolt ons - will only net you about 10-15HP max, as there is so much you can do with the OEM ECM and the breathing cababilties of the engine itself. As the mileage you are getting is very low for type of bolt ons you have - this points at something that is causing your engine problems, which is likely costing you some power.

To really open up decent power gains - you would have to literally open up the engine. Head work, P&P, increase CR, custom camshafts, EMS system, a real header, new intake plenum, larger throttle body, etc. Depending on how much time and money you want to invest in this - you could potentially get decent power gains. Some have hit 187WHP all motor with the 1ZZ-FE (over 215HP at the crank) - that is a pretty significant gain that is only trumped by forced induction or nitrous injection systems.

Most of the people that ran OBX header developed exhaust leaks - either right away or over a relatively short time. If the header is leaking, that could cause some of the mileage loss you are experiencing. Some people have found that running a ported OEM exhaust manifold yielded better results that after market header options. If you are running without a catalytic converter (note: you'll only gain at most, about 1 HP advantage, not worth the hassle and confusion to the ECM) - I would consider a PPE header. It is pricey, mild steel variants run around the $900 range, stainless steel ones run up to $1800. But you do get what you pay for in this market - equal length is only one part of the equation, stepped primaries, proper exhaust scavenging, etc. also need to be addressed. If you have other options, I would highly recommened not running any OBX product. Their quality control has really sunk over the past several years, more interesting in turing a profit then providing a decent part.

As for head work - there are a few good books on the subject. There are plenty of examples online, both good and bad. If you want to do it right, try finding some texts from author David Vizard - pretty much the authority from SB/BB pushrod engines all the way to DOHC, high revving, variable valve timing and lift import engines. Porting is something that is part science and art - not something that one could learn from textbooks, no matter how well they are written. If you want to try your hand on them, best way is to experiment on spare, junk heads. The 1ZZ-FE has some unique features in its heads that will require some "novel" work - best to send in your head to be built up by a shop or buy one outright. That way, you can always go back to stock, if you plan to trade the car in or need a spare in the future. Monkey Wrench Racing (http://store.monkeywrenchracing.com/) is a well known retailer that caters to the 1ZZ-FE engine. They have a good listing of aftermarket performance accessories for the 1ZZ-FE engine and will entertain questions from anyone needing some information.

The O2 simulators are a tricky piece to deal with. That's why I wouldn't even recommend removing the cat in the first place, as it doesn't net you much power with a cat delete and you end up with a CEL staring you in the face all the time.

As for what you need, depends on what code the car is throwing up with the exhaust you have. Could be a P0420 (pretty common) - in that case, a simple O2 sim would work - some decent ones are URD O2 sim runs about $80, I would skip the ones on eBay - try www.O2simmulator.com - as most of the eBay ones are rip-offs of these. Most of the ones on that site run around $30+ about the same as eBay ones, but know you have a place to communicate with if there is a problem.

If you have a P0141 - which indicates a bad heater circuit (downstream O2 sensor is heated) - then you'll need a simulator that has a built-in load to fake a heating circuit (generally some big load resistors).

Assuming you've already tried an O2 defouler - basically a sparkplug defouler (like a sparkplug extension - just look it up) - you basically drill a larger hole in the end, screw in the O2 sensor and the whole works into the exhaust. Supposed will give you enough leeway to trick the ECM thinking the cat is still working/still there.

Like I mentioned before, lot of goofing around for 1HP crank gain - if you want something that won't cause any CEL headaches and still flow well - consider a high-flow metal core catalytic converter or grab one off a Corolla XRS (OEM high flow cat).