The 3" difference in chain length is likely to be handled ok by the
RD, but you may notice a bit of droop in the chain if you go with
11t and a 30t CW or smaller. The more important factor is not
enough chain for the 34t cog and the biggest CW. If the chain
is long enough for the 34t (ie that was the original cassette) then
the extra chain length is not likely to be a problem. If the 28 or
smaller was the original cassette then there are several potential problems
including the RD and chain length. You presumably have some sort of
ATB set up so putting a smaller cassette on will just leave you with
a bit of a catenary in combos that are using small # of teeth.