After much research and consternation, I purchased the M4 carbon fiber slip ons. Overall, I am happy with my purchase. I will strive to provide a straightforward and unbiased review, but these are my opinions and yours may differ. Firstly, I would encourage everyone that purchases merchandise from STG to write reviews. Owner reviews are a great source of information and can be a significant aid in buying decisions. I like these guys, they have a great selection of merchandise at competitive prices, and it would really help if customers did not have to go to other sites to read owner reviews before laying down dinero for their purchase. OK, now my little rant is over.
Installation: Installation was a breeze, with the entire process taking a little over 30 minutes. The instructions were a little scant but if you have any mechanical ability at all, it is really a straightforward process. If you watch any of Brian Van's slip on installation videos you probably will not need to refer to the instructions. Its really simple guys (and gals).
Quality: The carbon fiber looks great and this appears to be a high quality product. I have been riding with these a few weeks and I have not experienced any manufacturing issues or defects. One thing I do not like is the springs that attach between the canister and the pipe. Its not that clear in the picture, but in person it is very noticeable. I may mod that in the future because I really do not like those springs. M4 should be able to design a system that would eliminate the need to have those obnoxious springs in full view of the general populace.
Sound: These sound really great, imparting a deep angry growl to what was once a docile sounding ZX14. I know people really want to know what these sound like so I am including a link so you can get a sound sample. Here it is. https://www.youtube.com/watch?v=R3_yyixOK3Y. It's amateurish, I know, don't worry you'll survive.
The rest: I have a Two Brothers carbon fiber on my ZZR600 and one difference stood out between the two. The Two Brothers pipe does not get hot at all. I can ride for an hour and put a bare hand on the pipe with no issues whatsoever. The M4 gets hot. Not as hot as my stock or a stainless steel, but there is a significant difference in surface temperature between these the two. The other issue I observe is that there is a noticeable deceleration pop when you close the throttle. Most of my research reveals that some deceleration pop is normal and harmless. This is just a little more than expected and certainly more than the Two Brothers slip ons on my other bike. This may be personal observation and it may not matter much to other people.
I also wish M4 offered these with black pipes like the retro drag slip ons that attach to the headers. That may be another mod for the future. Ill post pics if I do it.