The M1's are very smooth (probably due to the care taken to keep them matched together so well), and give a very significant boost in power and responsiveness all across the rpms as compared to the 275's.
I believe for the same amount of used power, the M1's are about the same or could be slightly more efficient as far as fuel mileage goes. Really was too close to tell. I did not see any drop in mileage with them, but they do have a "racier" feel to them.
What I did not care for was the way they raised the idle up about 50 rpm's which was probably due to better efficiency, but thats just my best guess. That affected the way the exhaust brake worked, and it took longer for the engine to drop to idle which was mostly just a little annoyance more than anything. It did bug me and just didn't feel right. Probably still, most people looking for a good towing injector, with good mileage, and significant performance gain would love the M1's, I was looking for a more mellow running rig off the bottom.
I went back to the RV275 and am probably going against the grain here, but have to say they have been the best bang for the buck (and best overall driveability) out of any injectors that I've tried.
As far as purely mileage goes, I just took a 750 mile trip (no trailer but started with 100 gallons of fuel) and averaged 18. 6 mpg overall. That was about 80% hwy & 20% city with the A/C on, so my mileage still isn't quite what I was hoping for, and may have been slightly better with the m1's, but straight out speaking, the 275's are VERY hard to beat for their price, if you are not looking for a race truck. Their driveability is hard to beat and thy run coooool (egt's).
I still have the extra fueling box, so dont have any problem with having enough power with the smaller injectors. There is still enough total fuel to overspeed the stock turbo.
It is a tough choice for sure on which way to go.
The other option I considered is going simple and sweet. Mach 1. 6's with no added electronics except a boost fooler.
It does seem to boil down to what a lot of people have posted. If you are light on the throttle, you can get good mileage with just about any injectors, except some of the smoke hogs. .
I hope that helps and doesn't confuse the issue more. .