Loads of info here is you search.
In general terms most bikes suffer due to emissions / noise regs etc. and can normally be improved upon by altering fuel and ignition maps, and with the advent of "ride by wire" the throttle mapping of what your wrist asks for and what the ECU does the throttle maps can also be changed.
In the case of the 2010 - 2013 S10's Yamaha restricted them so below about 5k in the first 3 gears you could not get anywhere near full throttle, therefore restricting the bike, here you can see a dyno graph for 2nd - 5th gear - and how awful 2nd and 3rd gear are low down.
The third gear dip may not look too bad, but I had my bike on a dyno and proved at 40mph in 3rd you are losing about 25% power, I also found this very noticeable when shortshifting through the gears, exiting roundabouts or overtaking slow cars / trucks etc.
All flashes will (or should) remove these restrictions, and on an otherwise stock bike are probably much of a muchness - the main thing that makes them difference is likely to be the throttle maps, a fella here (AVC8130) does flashes in the US and has a large menu of different throttle maps for each mode.
The ECU-U Flash I started with had a mega aggressive S-Mode mapping which I found made the bike a bit jerky, it was fine in the UK, but on Alpine Hairpins made it difficult to smoothly meter out the power existing tight hairpin bends.
On my bike I also had fitted a set of Arrow headers which really open up the breathing - but also cause a surge just of idle that the ECU-U Flash could not sort out - and the flash vendors do recommend fitting a secondary system (Power Commander etc) unit if you have other mods done, or want to perfect the fuelling.....
As due to manufacturing tolerance no two engines are the same, and most injectors will have a tolerance of +/- 5% or so, and the Manufacturers have to ensure all bikes will run fine for years without blowing up - so they set them for the worst case which means most bikes run extremely rich which loses power and wastes fuel. They also run extremely weak in the closed loop area (below 5k / 50% throttle) to pass emissions regs - fixing this will also give a much better throttle response, but most flash vendors are unable to map this area of the ECU (and again would advise a power commander)
If you do want a lot more power you will need to ditch the OE headers, but the results are quite stunning, this is my ECU-U Flashed bike (with Arrow headers) Vs a stock bike without a flash, you can see it gains everywhere with a load more top end.
Apart from the poor off-idle fuelling and OTT throttle map (probably put in to give the impression of bigger gains than the flash really gave) the bike ran very nicely and was noticeably quicker and rev'd out a lot better.
I have since used CJS racing in Bristol to flash the bike removing the work done by ECU-U, I think this showed up two things, firstly not all flashes are the same, and secondly having a custom setup done for your bike on the dyno will give much better results, CJS mapped each cylinder separately as well as optimising the ignition, this was about 4-5 hours work on the dyno but the results are impressive, not only on the dyno, but on the road where the bike is far smoother and idles more cleanly.
They did say the ECU-U flash was good at full throttle, but part throttle mapping was not great, and there was a slight imbalance between cylinders so each one needed aslightly different map to get the bike spot on:
They also gave me a linear (1:1) S-Mode map (a shade more aggressive than stock) and removed the T-Mode 1st - 4th gear restrictions (even at high revs the power is restricted in T-Mode as standard) so this now feels a lot more peppy, but still noticeably softer than S.
There are several options for flashes, if you want a custom setup without the cost of a Flash + Power Commander + O2 Optimisers (required to fix closed loop with Power Commander) the only place I know who can do this is CJS Racing.
If you just want a basic flash there are several choices, CJS will do one for £199 (Postal service) to remove restrictions and improve closed loop fuelling or the full monty custom setup optimising all parameters for your bike for £425 (about half the price of flash + Power Commander + Optimisers + setup) ECU Unleashed have at least one UK dealer (Price was £350 when I had mine done) and Velox Racing do a flash by post service also for about £200.
Pays yer money and all that, but everyone who has had a flash regardless of vendor has been very happy, I personally would recommend a custom setup on a dyno having sampled the difference between a generic flash and a custom job.