I can't think of any good reason for YOU to change, unless YOU want to. At a 5.5 index from the courses you play, you are hitting the ball pretty solidly and consistently. If you enjoy your game and want to stick with your clubs, then do it. The SGI clubs might mask some slight imperfections, but at 5.5 they can't be too bad unless you are just a pure genius on and around the greens. If you hit a wall on improvement, and desire to improve, seek out a good instructor and take a lesson.
I have two old sets of clubs. The ones in my sig are pure blades and I am also using persimmon woods with them right now. I am not "good enough" to be doing this, but I am just in an old school phase right now and enjoy doing it. I like the look, I like the feedback, I like the feel, I like the nostalgia, even though it probably costs me 4-5 shots or more per round. I don't do it to "impress" anyone, I do it because I enjoy it.
During "scramble season" I play with some older Titleist DCI cavity backs, and of course metal driver and fairway. These DCI's I would class as at least moderate GI-they are full cavity back with moderate progressive offset. I find I am considerably more consistent in dispersion in both distance and direction through the set. I game the same wedge and putter with either set, but occasionally put a Ping in play instead of the flanged blade putter I most often use.
When I play in scrambles or team events, I feel I owe it to my partners to do the best I can and be as consistent as I can, so the DCI's are usually in the bag then. When playing by my self or playing my own ball in casual games, it is usually the old school set.