The problem isn't that Ultegra rear has a 27 tooth limit & the cassette you will use, it's the 20 tooth difference on the fron chainrings. If you size a chain for the large front cog & large rear cog, in mana\y of the gears on the small ring the chain will sag a lot. If you size the chain to small cog/small chaing ring, you rip the derailleur off somewhere in the large chain ring. An ultegra rear der can't handle the chain wrap needed on a triple.