The sites (Z1 and other car part sites) always say the NISMO springs have a 0.3"~0.5" drop, but if this
thread says anything, apparently its not noticeable... Who knows, I never got around installing my NISMO springs and I don't plan to since I've decided to get a set of Swifts. I'd say you should go for the swifts if you're gonna go for a drop.
MAYBE since the convertible is slightly heavier, MAYBE you may get a noticeable drop.