Hi, Currently I'm using a MBP 13" (early 2011 i5 2,3ghz, 8gb RAM, Intel HD Graphics 3000 512 Mo, SSD 512gb). As a hobby I do a lot of :
1) photo editing in Lightroom (Raw), I'm satisfied with the MBP performance
2) video rendering in FCPX (proxy format while rendering)
I would like to improve FCPX performance and use DaVinciResolve to edit/color grading.
All my pictures and videos are stored on external HDD (firewire).
I think to start with 27" i5, 8gb RAM... but still wondering which configuration to choose for HDD & CG

1) photo editing in Lightroom (Raw), I'm satisfied with the MBP performance
2) video rendering in FCPX (proxy format while rendering)
I would like to improve FCPX performance and use DaVinciResolve to edit/color grading.
All my pictures and videos are stored on external HDD (firewire).
I think to start with 27" i5, 8gb RAM... but still wondering which configuration to choose for HDD & CG
- RAM will be upgraded later, so 8gb will be ok
- HDD, it used to be easy upgrades in past but not sure if that is still the case with 2015 iMac? (display glue???) If so I will switch with a SSD.
- is basic CG 2gb sufficient for my needs? If so which one? (So far my MBP with 512 shared was very slow but OK with a lot of patience...) Or the the 4gb is indispensable?
- A little doubt about the i5... Is it possible to replace the CPU later?