Thanks for the suggestions. Going to your questions
1) Seeing: I've two telescopes that I use in completely different environments. The small 130P SW / AWB is being mostly used in a urban environment (SQM 18.2 on average) AND for short night excursions on the hills surrounding my medium sized city (SQM ranging from 19.4 to 20.7). The seeing here varies enormously, from abysmal to very good. However part of the challenge is using a small telescope in a urban environment and "push the limit". On the other hand however my other telescope is a Orion Xt10 and I use it in a mountain (Alps) environment, with sky that goes from - at worst! - from 21.1 to 21.5 with a very steady sky except in high summer. So you see, the I can't say "I've good enough seeing most of the time", but often I've a seeing good enough
2) Optics: As I've said, it's a 130p and a XT10. The mirror of the XT10 is good. The 130 is a less than 200€ telescope
3) Collimation: Yes, definitely. I check the collimation before every session, both with a cap and a laser collimator
4) Cooling down: Yes definitely. And before you ask, I keep all my EP very clean, I'm careful with stray light etc etc
I understand that a high end EP requires particular conditions to be used at best and without them the money you spend is probably wasted. But I'm trying to understand how much I can much the limit with the gear I have. Do I have to stick to XCel-Lxs or I can try something more sophisticated?
Well, the more experience you have with eyepieces, the more you will realize that the center-of-field images in eyepieces like the X-Cel LX differ from the very high priced eyepieces by a very very small amount.
Seeing is going to be 95% of the difference between most eyepieces, on axis.
It's at the edge of the field the differences are more apparent, and keeping the field size to 60° will keep the edge differences small.
If your XT10 is used without a coma corrector, then the presence of coma will even make those edge differences even harder to see.
But, the wider the field of view, the more apparent the difference will be between low and high priced eyepieces. Still, the wider the apparent field, the more coma will be evident, so there is some value in sticking to 60° if you like that field size.
And 60° surely gives you some nice wide true fields in the short instrument, so there is no particular hurry to get wider fields.
You may find 70° fields (or wider) to be a bit more enjoyable, though, especially in the 10", showing you more real estate, so to speak, at each magnification, and longer times between nudges of the scope.
And once the apparent field size starts creeping up, the larger the difference will be between eyepieces.