The first thing that is going to have to go is that big titanium exhaust. It doesn't sound too bad for a standard bike, and seems quite light untill you put it side by side with an aftermarket one.
Easiest is an end can. From about £120 upwards you can get anything from a basic alloy can (e.g ART) to a multi-coulored titanium race one (Quill - my fave). Each manufacturer has different criteria on whether re-jetting is needed. If it is a road legal exhaust, it is highly unlikely that the jets will need changed. IF it is a race exhaust, then it is worth considering.
As far as performance goes, a road exhaust will add little to the over all power delivery. A race can will add about 5bhp straight from the box. 5bhp is easily noticeable when driving. After jetting, another 1 or 2 bhp may be released, but the big difference is in the mid range. Dips in the power curve will be smoothed, and throttle response will be much improved. On top of all that, there is less chance of the bike running lean and damaging the engine. This will give a total of about 135bhp at the rear wheel - something even an R1 would be proud of.
Now, I get asked a lot of questions whether or not jetting is really necessary. There are no definate answers. More often than not, jetting will improve the engine response. My guess would be that you would be unlucky to damage your engine - but it is your choice. If you want to try it yourself, you could do worse than upping the main jets to say 157.5 or 160 (from 155).
Full Race Systems
If 5-7bhp is not enough, then a full system must be on the shopping list. One of the quality systems like Akropovoc will cost upto £1000 with jetting another £250 or so on top of that. But the results of up to 15bhp more than speak for themselves. However, there is a down side to this, and it comes in the way of poor low down running. It can be so bad that if the needle drops below 3k, or so, a slip of the clutch or a down change is the only way out. Also, at MOT time, the standard exhaust must go back on (same if you get nicked). Of course, this is also true for the race end can - but that is only a case of four bolts.
However, in this case, you really must get the bike jetted to avoid engine damage and get the best performance. Stateside (and lately UK too), the Factory-pro jet kits are quite popular. Recommended setting are;
The settings for the Factory-pro K-88 1.7 race kit is as follows:
1. install the supplied #40 pilot jets
2. #172 main jets on 1/4 cylinders and #175 for 2/3 with thesupplied emulsion tubes.
3. the main air jets (in carbs) must be drilled with the supplied drill bits.
4. i set the needle clip on the #3 position for the best mid-range pull.
Considering that most standard '9's put out something like 128bhp and another 15 or so is available by swapping exhausts, you could now have up to 145bhp at the back wheel. On a coldish day in November, mine actually made 140bhp with nothing more than bolting on an end can. So, depending on the conditions, 150bhp is not to far away. If you want more than that, things get a little more complicated.
You Want More
The principles of tuning any four stroke engine are similar. They are also pretty much beyond the average person. Basically, the target areas would be gas flowing the head, raising the compression ration, different profile cams, bigger bores, bigger strokes etc etc. Not cheap and not easy. I have done most of the above on cars and generally the money would have been better spent on buying a faster car.
Gas flowing the head may seem straight forward, but unless someone tells you where to take the material out from, it is as likely to be worse than better. The information for porting escorts and minis for example is pretty freely available. But, not so for the ZX. Simply polishing the tracks are almost certainly not going to reap any rewards. So, that only leaves a reputable firms. A little skim of the head and a bit porting here or there would probably not be too expensive and at £/bhp might be similar to fitting a full race system. Maybe someone out there has done this and can tell me?
If you want to see what is available and what it takes, try the link
Alternatively, go down the root of nitros or turbos. Twobros racing run a turbo'd ZX9.
OR, TTS have developed a big bore kit for the ZX9 - Lifts the capacity up to 1000cc. Should be enough to whip any GSXR. Learn more by reading the recent MCN article about the TTS conversion - HERE.
I guess there is one other point that may be worth mentioning. Gearing. It has become quite popular to drop a tooth of the front sprocket or add a couple onto the back. This will make the bike quicker at the expense of top speed. Also, if you are a wheelie merchant, it may be just what you are after. And it costs virtually nothing. The only down side is that the speedo will read wrong. Go to Halfords and buy a digital cycle computer. Not only will they read up to about 200mph , but be more accurate and give other usefull info like the maximum speed achieved on your run. Good evidence for the coppers when they pull you over!!!!