TurbogRAMpa,
To answer your questions, the injectors should not affect milage, however, your right foot will! The injectors increase low-end power, which works well for towing. Your ability to tow at greater speeds due to the increase in low-end power will lower your overall gas milage, but minimally. The outcome is that you are able to tow at higher speeds, which requires the use of more fuel that the injectors provide. Unloaded, there should be no noticable changes in milage.
Yes, you will need to get the sealing washers to install the injectors. You can purchase those at any injection shop for no more than $10-15. If you want, you can reuse your existing washers by cleaning them with a Scotchbrite pad. I have done this many times when changing and testing my injectors. However, I would contact Piers (PDR) and order a kit. It is better to be safe than sorry.
After the installation of the injectors, you will most likely have to move your fuel plate back to control egts, due to the injectors providing more fueling throughout the power curve. Please, whatever you do, DO NOT go avbove 1300 max egt.
One advantage to the larger injectors is that it allows you to run the 16cm exhaust housing without as much low-end lag that you would get running the smaller injectors. This was the only way I could run the 16 and still maintain decent low-end response.
I want to make it very clear that the marine injectors, whether 300 or 370s, will smoke and haze. One person's definition of smoke and another's are two different things. If you are against any smoke, you are more than welcome to pass on purchasing the injectors. That is the reason that I am selling them... because I don't like any smoke. I will miss the power they added, but that is my preference.
If you'd like, you can call me at work tomorrow (I e-mailed my phone number to you) to discuss advantages/disadvantages of the injectors. I could also provide some installation tips if you are interested.
DBR