Any benifits of upper level bibs for a fat long distance XC biker? Skinny road bikers are all praising their Castelli/Assos bibs. Specially long distance riders.

I'm 188cm/106kg and my normal ride is something like 100-120km, 2500m gain, 8-9hours in saddle almost not-stop.

I'm quite happy with cheap fake Castelli bibs from China (17 euros or so). I was that happy with some lower level components until I replaced them with upper levels. More joy actually.

Its easy to get reviews for components but not that easy for clothing. Specially clothing dedicated for long distance XC.

For example I can get Castelli Soprasso tights for around 125euros w/ discount which is not much higher then wiggle's dhb brand.
You'll only benefit from better bibs, insofar as they fit as they should. Given that you seem content with your current set, start with the same size when making a new purchase.

For the differences...

Start by checking the stitching all the way around on the new ones that you're interested in and compare it to those that you already know. Quite a bit of the new school stuff really has really improved stitching, or even eliminated it altogether, by moving seams around and flattening things out to mitigate chafing potential (nipples, inner thighs, top of your shoulder, chamois edges). 

The rubber, or rubber-like, sticky gel that is inside the bottom of the shorts that will be at the bottom of your thigh, just above your knee, will keep your shorts from riding up. The effectiveness and durability of this gel is important because nice shorts that creep up into a wrinkled mess at the top of your leg can wear a hole in your shorts, or your skin. Some shorts have dots or segmented lines of this discreet help while the most effective ones have a solid band that is not woven into the stitching.

Newer, higher-cost, bibs might even have little pockets on your thighs for making food super easy to access while you maintain your riding position. Similarly, there might even be a strategically placed headphone cable pass-through sewn in.

The most significant difference, however, is likely the chamois, or the pad that you'll be relying on for those long rides. Turn each pair that you like inside-out to get the best look and then feel for the differences between the brands and the models. The higher quality padding tends to be more dense while also being more comfortable against your skin. It is likely perforated (lines, slivers, or small sections of slightly less material) to properly fold at the very top of your inner thigh where your seat can rub quite a bit. The padding can consist of multiple densities to accommodate the different pressures that you exert on your seat. Your "sit-bones", or the two bones that ALL of us, regardless or sex, weight, riding style, or skill, expose when we sit down and lean forward to hold on to the handlebars will hopefully line up well with the strongest part of the chamois. There may be an unbelievably discreet layer of gel within the pad that can really help out. Those with the gooey, thick gel will have you sliding around on your saddle and add bulk.

Good bibs are a must. Everyone has a budget, so find the models that are within yours and get some that are nice and tight since they'll stretch out a little over a couple of years. Always hang them up inside out so the chamois can breathe and wash them with a light amount of soap. Don't allow them to host any bacteria or gross out whomever is on the bike behind you! Quality bibs will easily last a couple of years of daily use if you care for them well.