Well, there are mods to improve performance and there are mods that prevent catastrophic failure due to Ford's corner cutting on the 1998 and later Contours and Mystiques. The mods you should consider for maintaining performance are:
1. If you have a 5-speed, pull the shift tower assembly and send it to Terry Haines at HMS to do what is called the "keyed" mod. The towers are notorious for breaking a bolt inside the shift tower. The head of the bolt drops into the interior of the transmission and often gets caught between the gears and the transmission case. This results in destruction of the case and requires replacement of the transmission. I won't quote the price of this since that is Terry's business.
2. Modify how hard you try and take off in the car to avoid wheel hop. Ford cheapened the differential inside the transaxle in 1998 making it ~ 20% weaker than the 1995 - 1997 transaxles. As a result, numerous owners have destroyed their transaxles due to the spider gears fracturing and blowing out the transaxle case. This modification costs you nothing.
You can do mods to the roll resistor mounts on the front and rear of the engine which will help lower wheel hop. Not expensive to do.
3. Still pertaining to the 5-speed, it's OK to romp on it once in a while when it comes to shifting quickly. However, if you plan on driving it hard (slamming shifts) often while performing modifications #1 and #2, the synchronizers won't hold up very well and you will end up with grinding gears during shifts (time to rebuild the transaxle). This modification costs you nothing.
Basically, the MTX-75 transaxle is just not very durable; especially ones made in 1998 and later.
Other preventive mods:
4. Put some sort of an oil catch can into the PCV system to significantly reduce / stop build-up of oily carbon residue in the intake manifold assembly. The Duratec V6 is notorious for building-up nasty deposits in the manifold which restrict airflow and can cause the throttlebody and secondaries to get stuck closed (moreso on the secondaries). Many owners will make their own filters but you can also buy filters that are normally used in air compressor systems to capture oil mist. Grainger is one place to buy these online. This can cost as much as $75 depending on the contruction of the filter housing (polycarbonate vs. steel). The biggest headache is where do you mount the filter in such a congested engine compartment.
5. You may need to pull your waterpump and see what kind of impeller it has. The earliest Contours used a black plastic impeller which was prone to fracturing resulting in the engine getting destroyed from overheating faster than the driver can react to shut it down and cool it off. Ford did change to a beige-colored plastic impeller which fixed the problem. I do not recall the exact year this change was implemented. My 1999 SVT had the beige plastic impeller. You can also buy a waterpump that has a metal impeller to total eliminate the worry about any plastic impeller breaking (pretty cheap mod).
And then there is the spun connecting rod bearing problem on 1998 and newer Duratec V6s (more common on the SVTs)....
Are you sure you have deep enough pockets to play the mod game? The general rule in speed is not how fast you want to go, it's how much you want to spend. In the case of 1998 and later Contours & Mystiques, the cost to go faster can escalate more rapidly than a lot of other cars because the drivetrain is just not very robust.
By giving you all these warnings, I'm trying to get across the principle of "Do the mods that make it last before you do the mods that make it fast." You will likely suffer much less frustration in the long run that way.