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Hi Charles,
The C8 is issued with a standard 1.25" diagonal & adapter that fits onto the rear of the scope (called a visual back). This adapter is removed & a 2" adapter is installed allowing for a 2" diagonal to be used. This goes for all Meade & Celestron SCTs. While the 5" & 6" SCTs can take a 2" diagonal, hole in the back of the scope isn't large enough yo allow for the best use of 2" eyepieces.
AVOID these packaged eyepieces. Not only are they way overpriced, the EPs are not worth the trouble of "upgrading". There are much bettter quality eyepieces that can be had forexperiencedmoney. I know of three eyepieces, along with a 2" diagonal that can be had for pretty much the same cost as that kit, & their quality is significantly better.
Coma is a function of fast Newtonians. The faster the Newtonian, the more coma will be visible along the edge of thr field of view. This is also more significant at low power. But, there are contraptions called coma correctors that fix this. This, for me, is only really importfor photography. I don't really find coma all that significant, even in my 8" f/4 dob that I love for the expansive wide fields I can get with it. But this is a personal preference.
So, yes, coma is more prominent in an f/4 scope than an f/5 instrument. But, don't tie your knickers in a knot over it. It can be dealt with. More important is finding a scope that best suits you.
I suggest you get to a star party or astro club to see these scopes in the flesh & in action.
Mental.
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