I am planning on getting a Cube Reaction when they're released next year. The spec's as follows:

Frame: HPA 7005 Advanced Hydroform Triple Butted
Rock Shox Sid Race 100mm, Motion Control, Poploc / FSA Orbit Z headset /
Syntace stem F139 Handlebar / Syntace Duraflite 2014 handlebar
Shimano Deore XT front mech, cranks / Sram X9 rear mech / Shifters: Sram X9
Formula R1 Magnesium Disc Brakes(180/160mm)
Wheels: DT Swiss XPW1600 (which I assume means they are about 1600 grams)
Tyres: Schwalve Rocket Ron(Front)/ Schwalve Racing Ralph(Rear)
Saddle: Fizik Tundra MG
Seatpost: Syntace P6 
• Total weight . 10.4kg

I would like to bring the weight of the bike down further to somewhere around the 9kg mark if possible and it seems that most would argue that the best place to lose some weight would be reducing wheel weight / my tubby ass! (I'm only 64kg)

I have the option of upgrading all the SRAM X9 and Shimano XT parts to XTR which would reduce the weight and increase the price by £300 or stick to the XT/X9 parts and get a light wheelset (still deciding whether to get handbuilts or a Crossmax SLR). Now, that would bump the price by about £500-£600.

The bike retails for £1.5K at the moment and my budget for improvements is an additional £600-£700.

I'd be interested in your suggestions / advice on which course of action to take or perhaps if you have a completely different idea, fire away.

Cheers

Take your extra £700 quid, book a biking holiday somewhere nice and hot. As for reducing weight, have a huemungus shite just before the ride, jesus  you spend 1.5K on a 10 kg bike and you want to lighten it?

Ouch Mick!

Im also looking on that bike ,
and i think if you want to do it ligther you can find another finish kit and wheelset.
i know a wheelset to about 800£ at only 1100 gram. there is half a kilo saved...

I think Mick had this sussed.

Mick's spot on. Ultralight, trick bits tend not to work as well. OK for lottery winners, but no point in saving a few grams and walking home because something's broken.

the puncture demons will be after you now nick