Adding to Bojan's points.
* An f/7 mirror would mean a smaller secondary mirror is possible to be used.
* % smaller true field of view isn't that significant from f/6
* From fabrication point, it is easier to produce a fine figure in a slower mirror than faster, but th f/ratio difference here is not that great. You will eant to KNOW that you ate getting a FAR BETTER f/7 mirror to justify the trouble. You will want a test report & know how to interpret the report results, or know what to look for in a star test. A mirror with a Strehl ratio of 0.98 is as close to perfect as possible, or a wave ratio of 1/8 or smaller.
* Hanging a bunch of extension tubes off the focuser is not the way to accommodate the longer focal length - you will introduce a huge amount of fles into the optical train. You really need a longer OTA. The new mirror may not actually fit the existing primary cell! I've seen this very often. For instance, Synta mirrors are all slightly over-size and won't fit in a GSO cell. You may need to make a new cell for the primary.
* From a planetary point, f/7 and slower is often considered more desirable than a faster ratio. I know of a 15" f/8 mirror, and damned if it is not tempting for me for making a lunar and planetary scope!!!
* Photographicallly f/7 is not really a big deal with today's cameras for imaging DSO's.
Unless you can determine a significant improvement in quality between the two mirrors, and this means testing both mirrors, really testing and not just hearsay, it isnot worth the change of mirrors. I am assuming your mirror is a GSO. On a whole their mirrors are good. But they can also throw up astonishingly good mirrors too. You need to know how to identify what you have, and not assume that because it is a GSO that it will be ordinary. That would be a mistake. Same with Synta mirrors. You might be surprised.
If you want some pointers on what to look for, just ask. For a novice it can be difficult to interpret star testing images, but there are some tips.
Alex.