The new generation of BOP stacks (6th and 7th generations) combined with the potential capping stack weight who now has to be considered is the main driver for the 2" CP.
Be carefull as this requirement for 2" is not over the entire Conductor but generally just for the WH extension and first joint below, as most of the studies generally show that stresses are concentrated within the first 15m below mud line... (This can vary slightly according to soil properties on locations but is unlikely to go much deeper than 20m bml.)
Hence, you are ending with WH extension + 1 or 2 jts in 2 ", XO to 1" for the remaining of your string.
The 10x DNV safety factor is not shocking and is a common industry safety factor for structures (we are dealing with a lot of uncertainties here, such as self frequency for BOP stack, riser assy...).
The resistance to fatigue of your connectors on CP has also to be properly evaluated (Companies like 2H/Stress/Atkins/Aquaterra, and I am probably missing some others) can give you good recommendations.
Recent article published in the WO http://www.worldoil.com/March-2013_drilling_riser_fatigue_monitired_in_real_time_aboard_drillship.html
can be of some interest for you.
Hope this helps,