The -5 Hellcat was substantially different than the -3, yet the overall appearance remained the same. The differences in that regard are confined to a redesigned cockpit canopy with fewer braces, the deleted rear-vision window, and a more streamlined engine cowling to reduce drag.
The greatest changes in the designe were those that were implemented to meet a Navy demand for increased performance and improved roll rate:
Water injection
Redesigned, lower drag cowling.
New, non-specular, paint job with a factory applied coat of wax to reduce drag.
These changes yielded an increase in top speed that, depending on the sources cited, varied between 401 and 409 mph, up substantially from the oft quoted top speed of 380 mph of the -3. Other changes in the -5 was the attachment of hard points for the carrying of 1000 pound bombs, or three 150 gallon fuel tanks (a combat configuration that was rarely used), and the mounting of six rockets. The horizontal stabilizers were also strengthened, giving the Hellcat to safely reach diving speed redlines as high as .80 mach, according to Grumman engineers.
Regards, Shuckins