Hi,
A newbie here that is a Carboncore owner. I have the Y6. I bought it as a 650mm but I now also have the 950 arms too.
First off, let me say that I have no connection with Carboncore other than as a satisfied customer. I can say that the Carboncore kits are beautifully made and everything fits. No drilling or filing required.
Regarding the mention of MT3515 and MT4008's on their website - this is not a recommendation for motors to use, it is purely a statement that they will fit if you want them to. Cameron who runs the company tries not to make any recommendations regarding motors and electronics. I recently email Electriflite who are mentioned on his site and got the following recommendations:
T-Motor MT2814 770Kv 4S lipos and 12x4 props should give you a payload lifting capability of around 2Kg to 2.5Kg (4.4-5.5lbs)
T-Motor MT2826 550Kv 5S lipos and 13x5 props should give you a payload lifting capability of around 3Kg to 3.5Kg (6.6-7.7lbs)
T-Motor MN4012 400Kv 6S Lipos and 15x5 props should give you a payload lifting capability of around 4Kg to 4.5Kg (8.8-9.9lbs)
Those payloads are in addition to the weight of the machine.
When I bought my kit I bought various bits from all over the place. If I had purchased it all from one place then the errors in my spec might have been spotted and money saved. The first mistake was that I calculated that you could fit props of over 18" to the Y6 650 so I used ecalc and decided on T motor MN4012's (second mistake)with 16x6 props. They fitted fine on the upper motors but I had forgot all about the undercarriage! So plan B was then either to go to smaller props (Low kv motors are more efficient with big props) of fit the 950 arms. The arms won.
The arms are a straight swap and the weight difference is only 10g per arm. With a Naza and 2x 4S 4000mAh it flys well in the hover and in gentle flying. Hard climbs, accelerations and descents cause it to wallow as the Naza tries to keep up. Large motors with large props have a lot of inertia so they do not speed up or slow down as fast as on a smaller machine (eg Discovery). At the moment I am only flying with a GoPro and/or a Nikon J1 so it is quite light. I have tried adding another kg of ballast and it is much more stable.
In hindsight, since I am not going to be carrying the weight of a 5D etc, it would have been better to have gone for the smaller motors from the start. I have now ordered a set of MT2814's and will downsize to 11 or 12' props. I'm also thinking of swapping the configuration from a Y6 to an H6 as the lower props on the Y6 are very close to the ground and I can see myself breaking a few props!
In short, Pick your motors according to your payload rather than just for efficiency. Changing from a 650 to a 950 is simply a case of changing the arms. All other parts are the same. To change from a Y6 to an H6 (or octo) will require a few more part (frame plates, ring mounts, arms etc) but is still easy to do. Spares support is very good.
For carrying a NEX5 I would start with the H6 650 and MT2814's and then change the arms if you really feel the need.