Before you do this, realize that adding a left muffler/mid can be more work than you realize. I've done it. I do like it. Like the power band. But it took some time to get it the way I wanted. (Different side stand/modified center stand/left hanger/left Mid that fits close, etc etc etc...) I can offer suggestions if you add the header. The exception to that extra work may be the Delkevic set up. (Parts may be in the Delk. kit ??)
Just a FYI; Moving to a 4 into 2 can involve a lot of work. I used a 2008 ZX-14 header and had to modify; side stand/center stand/left muffler mount had to be built/left mid pipe was modified for a better fit.
Lots of COG Members have installed the Delk. Slip-on's and love them. On the headers, fewer have installed them, but I've heard no complaints from those that did. As far as I know, all did the 4 into 1 system. You may be the first to do a 4 into 2.