I have considered a growth of around 15% OCF on FY14 for FY 15....

I have considered a growth of around 15% OCF on FY14 for FY 15. The report by Baillieu holst suggests OCF in FY 16 to be > 40 but for FY 14 is predicted to be the same as FY 14.
However, we can see that in FY 16 the FCF increases as capex decreases.
https://www.baillieuholst.com.au/publishedresearch/2014/voc_28aug14.pdf

I haven't really closely looked at the what the combined entity looks like ( probably because I don't want to be disappointed if it doesn't go through and I don't want to bring too much speculation into the picture for myself). ..but your numbers seem about right.
Although in mergers if we look at valuation from the perspective of a Cash flow model ,will essentially be FCFF * (ROC-g)/WACC-g .  The combined entity should according to theory result in a lower WACC due to synergies and cost advantages. Thus even though growth may slow this is compensated somewhat by a lower WACC. Qualitatively, I see competitive advantages increasing which means that although the ROIC may fall due to factors you mentioned, the ROC should still beat WACC and the moat will ensure that the ROC's remain fairly high for a prolonged period of time allowing compounding to work its magic for investors.