I have arrow headers - the weight saving is minimal on a bike that ways upwards of 250kg, you just won't notice it.
The power increase is significant, about 6BHP on mine, and with the Flash is now up about 10BHP on stock, I suspect the flash alone is 2-3 BHP at the top end.....
.... But the Flash is about removing the low down restrictions in 1st - 3rd gear which makes a huge difference, for example my bike has more than 25% more power at 40mph in 3rd, this is why the Flash is the number one mod for most, it gives a stack of extra Ooomph where the bike gets used a lot, especially if you ride really twisty roads.
The Flash does not 100% sort out low speed fuelling, neither ECU-U, nor Flashtune have managed to sort this, others know more about this that I do, buy I think there are some areas of the fuel map they either cannot adjust, or only have limted adjustment off and this prevents them sorting the surging that occurs at <5% throttle @2,000rpm.
This means you either have to live with it (not difficult to ride around) or go to the expense of a Power Commander which then makes the extra bit of top end power they give very expensive, if you have the budget do the lot as the top end increase is noticeable, mine no longer feels like it is running out of puff towards the redline.
The surge still annoys me a little, but on open roads is not an issue, I have just done a 1500 mile trip and had 5 days in the Alps on tight mountain roads, not once did I find the bike surging - the only time it hits me is crawling along in traffic, but if I had to do my daily rush hour commute on the S10 I would either have to fit the Power Commander or remove the headers.
Flash = Best Bang for buck, and where you will notice it most
Headers = Fair top end gain (but very costly if you want it setup perfectly)
End Can = May improve sound / looks but do not expect any noticeable power gains