I'm getting ideas together for what's going to happen to the Cinq over the winter period, now I've got to audio. I'm going for a Alpine iDA-X001 head unit but now I want to spec the rest of the system, however I want to try and keep the weight down to a minimum.
What I do know is the front mount 10cm speakers in both my cars aren't good enough but I'm not looking for great audio just something that's got a reasonable response across the frequency range.

I've had one suggestion of Diamond Audio D362.5i for the speakers running off the head unit but I'm just clueless, anyone have some suggestions?

The whole weight thing is a bit of a pointless concern.

Do you only ever put £5 worth of petrol in your car to keep weight down?

The Cinq has 4 mounting lugs for the speakers IIRC, how about a set of these up front (speaker should fit behind standard grill, tweeter in door):

http://www.caraudiodirect.co.uk/dynamics-sd42-p-5521.html

and those Diamonds in the rear shelf.

I have no problem increasing the weight as long as it's useful but I see no point in adding 25kg of audio equipment if 10kg will do me fine. So maybe excessively heavy would have been a better title. In terms of weight, it all adds up be it reduction or increase lets take a little look at the bit's n pieces i've taken off the Sei in terms of weight
2.5 + 3 + 6 + 4.5 + 7 + 11.5 + 6 + 4 = 44.5kg or 5.5%
that's around the equivalent gain of a performance back box.

Anyway instead of going for those two in combination would it be better just to go out and get a 3way speaker set like Rainbow SLC365 or Infinity Kappa 1.7cs or am I putting to much trust in companies getting it right?

If you were to go with one of those 3 ways (of which the Rainbows would be my choice) ideally you would need to amplify them as the headunit just doesn't have the balls.

Also, ideally, you would be better mounting the midbass in the rear quarter panels so not to drag the stage too far back. You could do a door build for it but that would defeat the object of what you're trying to achieve!

So the Alpine iDA-X001, Rainbow SLC365 & Mutant NW280SP (which I have now) would be a good place to start?

Sure would.