The majority of review state that the rk47 chronos in at about 360 with a 0.2g.
The use of a 0.25g bb would only drop the FPS down to approximately 320ish fps, which is still more than reasonable. With the correct hopup settings a 0.25g bb could travel a fairly decent distance, compared to the use of a 0.28g bb which would drop the FPS down to somewhere along the 300 mark, which in my opinion is still a decent velocity. Coupled with a good hopup setting it should still achieve a fairly decent distance. I personally would not go any higher in BB weight than 0.25g or 0.28g in the stock rk47.
"Its only a little bit on fire"