Anyone got any idea what effect linking the downpipes like this has on performance?

from this link.

http://www.torquetechexh.com/whichsize.htm

Balance Tubes (or 'H' pipes) help to even out the pulses in the exhaust gases and balance the backpressure between both banks of your engine. They not only help improve horsepower, but sound as well. To find the right location to install your balance tube, apply a strip of paint down the front portion of the exhaust pipes. The balance tube should be placed where the paint burns off (bubbles) on the exhaust pipes. Generally, put the balance tubes as close to the headers as possible. The balance tube diameter should be at least 75% of the diameter of the pipes with which it connects.

"Balance the backpressure" ??WTF ?

So it's a way of making the best of a poorly designed exhaust. system !

That link seems to lead to a bunch of idiots........

From what I have gleaned from the www, link/balance pipes are mainly used to quieten the exhausts by 'sharing' the energy in each pulse between each silencer. That should also mean that by 'sharing' the energy, it must be letting the stinky gasses out easier. This theory is for V engined cars running two main pipes and two banks of silencers. Quite what those link pipes are for is beyond me. I suppose that they are in fired opposite pairs so that would make it sound nicer!!

A correctly placed ballance pipe can "sometimes" add power, but will always improve the silencing. They are usually used on V6's and V8's with twin pipe systems, and possitioned just behind the bell housing, they can be a fair bit smaller than the actual exhaust pipe size. I have never seen them fitted as per the picture, and can see no good reason to put them there. Cheers Ray

could they be there not to balance the exhausts but to hold them together????

Ie the same as welding a strip of steel between them??

They are genuine Kawasaki pipes (are stamped Kawasaki Heavy Industries on collector) Guy I got them from said they came of his racing Quacker and are a factory fitment.

After asking on ZX9R.net, it seems that this header is the standard one from a ZX9R-E and is a popular upgrade on the 'C' gives increase in power in mid and top end, with no downside at bottom end

there could be a several things going on which could confuse this discussion. Firstly the jetting of the carbs on the E series onward is different in so much as the outer two cylinders have a different jet size to the inner two, which corresponds to the exhaust link arrangement, not sure why this is...anyone comment ?. The increase in power from the C model is most likely due to the increase diammeter of the later E series exhaust ports and down tubes, and may not be due to the links at all. One reason not already mention is the effect of puse scavenge which is used to help clear the gases from the port exhausing. However, I'm no expert and could not decide why these were used. My ZX9 E motor runs without links on a fabricated manifold, and with a stage 1 dynojet kit, pipercross filter and IXIL can made 147 BHP and 72 lb.ft torque at the rolling road

As far as I'm aware running different size jets in the inner cylinders is to keep combustion temperature sightly lower.
Seen more commonly on air cooled lumps.