That method is inherently limited because it involves having the magnetization in the ecuatorial plane for a long period, thus suffering heavy T2 relaxation losses. If the radiation damping/water signal is a problem, it is possible to either use a typical STE derived sequence like Oneshot with a first pulse with reduced angle or combine an STE sequence with a water suppression element like in PGSTE-WATERGATE (or even with presaturation).

Additionally, for precise diffusion measurements of large molecules it is necessary to have a gradient coil that can safely (avoiding temperature issues) provide strong enough gradients. For that paper they used a coil that provided up to 0.40 T/m, which is still the standard for many spectrometers currently in use. Now commercial gradient coils reach up to 40 T/m.

In the following paper (http://dx.doi.org/10.1021/bi801933c), authors measured, apparently with a PGSE sequence, diffusion coefficients of 10^-6 m^2/s, which according to their centrifugation data may be of the order of 300kDa (it is not clear from the text). It should be feasible to measure larger molecules with PGSTE derived sequences.