Remember, the key for most countries is not to match the Brits in Heavies. You're not going to be able to out-heavy the British, just avoid the -1, from the light ships. It's the Brits that have to take special care to build Heavies, so that as it acquires more enemies, it doesn't give the other side a +1 from local superiority.
The new costs are actually slightly anti-British. 90%+ of the time, none British builds should be LS, while the British needs to build ~50% HS at longer time and higher cost. Individual games may vary, of course.
Actually, there still is a good reason for GB to build light ships: Piracy. If the piracy rules are not in force, or are IN force, but using the current (broken) implementation, you are mostly correct. However, if the game was fixed so that GB could actually interdict French American trade, then building light ships makes a lot of sense. In fact, just having piracy turned on allows GB some use of those light ships (once the French navy goes glub-glub): She can target non-allies' regular trade.
NOTE: Technically, she can target even allies' trade, but since nobody else in the game can do it, everybody will know who it was. Not a good way to get on the good side of your ally.
At LAST! The greatest campaign board game of all time is finally available for the PC. Can my old heart stand the strain?