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you could be okay .. play around with B tension and limit screw .. or this: http://www.wolftoothcomponents.com/collections/small-components/products/roadlink
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It very much depends upon your frame geometry. The combination you have may work on another frame and not yours.
If you get the chain length correct i.e. it is long enough, the worse that will probably happen is that one of the derailleur jockey wheels will rub on the largest cassette sprocket when in the lowest gear.
Just got the new Tiagra 4700 rear derailleur. Noticed that contrary to the specs on the website I bought it from the max sprocket size is listed as 28 Tooth rather than 30. The tech docs back that up as well: http://bike.shimano.com/content/sac-bike/en/home/components11/road/Tiagra4700/rd-4700-ss.html
Am I going to run into trouble with a 30T sprocket, or is there likely enough wiggle room to get away with it? Feeling lazy so don't want to return it and get the long cage version.