My experience says your CPU will be cooler with 2 fans and duct than with what you have now.
Not saying you need to do it. Just that segregating heated exhaust from cooling intake air keeps inside of case cooler.
Duct in first red Phanteks pic was out top of my R2 (second pic is out back of R2). With exhaust out top I could unplug all fans and things never went above 70c... and that was after 25-30 minutes of rendering with CPU graph drawing a straight line at 100% load... and when rendering ended it would take about 10 minutes for everything to cool back to normal. With fans running temp maxed out in about 3-4 minutes at 50c with fans running 950-1050rpm... and when rendering ends it only takes a couple of minutes for cool back to normal.
Obviously with fans off not just the CPU/cooler got warmer but so did mobo chips, GPU, HDDs, etc.
R2 has 2x 140mm vents in top that can be open or closed off with sound dampening panels. These were both open with duct out top so cooler was acting as a chimney.. and so was case.. moving air up with convection.
All of above was with i7 980. When I changed mobo from X58A-UD3R to UD5 I changed from Geil RAM to Crucial which is shorter and I could mount cooler facing back... Which I did and was able to replace the top sound dampening panels. I also cut out the back case vent grille and left the mobo back panel off to allow NB heatpipe radiators to vent easier. Can't say it made it any quieter but now I can pile more carp on top of case
Even experimented with boxing in bottom of cooler so the airflow from bottom of front fan was forced out only through the NB heatsink radiators. UD5 has 3 of these around CPU socket; bottom, back and top. Doing this only lowered NB temp from 51-57c to 47-53c. If.. no, when I have cooler out again I may not use it again. Guess it depends on if I have any problems with it staying in place while removing/replacing cooler.