The distance from the front wall (the wall behind them) affects bass performance and soundstage depth. If you have to slam them against the wall, you might have to use port plugs. And if you slam the against the wall, you will have a 2-D rather than 3-D soundstage.
Your kitchen should not be a problem. The M2OW should be more than adequate and if you want to play louder and deeper, the M5OW will give you that plus cleaner mids and highs to boot. You always have the 600 to back them up. If you really like the M50 sound, get M3OW.
Your living room set-up is very non-ideal as you already know. Given your listening distance and speaker separation, the imaging and soundstage is virtually non-existent. LFRs won't help. Sure, they'll go loud and clean. You may actually be able to get better imaging and soundstage with M2 or M5 OW or bookshelves.
If you are enjoying your M50s, I think you should just stick with them and put the money into side surrounds and kitchen M2, M3 or M5. If you can ever make the front of your room "better", then I'd say look beyond the M50s.
If you've had enough of the M50s and feel you just have to upgrade them, the M60s will give you better everything. But that "better" will sound very different compared to the M50. More accurate, precise, and more detailed. Some don't like that and want sound to be "mellow".
