The naval 6" BL gun that Hildebrand got in exchange for her 4.7s weighed 16,875 lb (7,654 kg) each. (Thanks wikipedia, just in case I appear some sort of gun nut. ) (http://en.wikipedia.org/wiki/BL_6_inch_Mk_VII_naval_gun
The 4.7s weighed much less at only 4500lb (again thanks wiki)( http://en.wikipedia.org/wiki/QF_4.7_inch_Gun_Mk_I%E2%80%93IV
) so the weight increase per gun was over 12000 lb or for 8 of them was almost 100000lb, which is about 50 tons. I am sure there were cases where the extra weight of guns destabilised some of these ships, particularly if they are mounted high. However, some of these ships were dual purpose, passenger and cargo, and of course there would be none of that in them so it may be that the added guns had little effect other than to require some ballast removal.
Several ships seem to take in over 100 tons of coal at a time, so again the weight of guns may be insignificant.
Just in case I sound as if I might know what I am talking about, I confess now that I dont, so should probably shut up now!