That is irresponsible of your builder and is a warning sign about their knowledge, quality and integrity. Reduction from 8" to 6" alone is a problem. It will reduce air flow and so the effectiveness of your hood and it will increase hood noise as well as the amount of electricity used. So increased cost, more noise, less odor, gas combustion by-products and other effluent removal. Flex duct creates two problems; It's a major fire hazard and it increases static pressure (reduces air flow). From a static pressure standpoint 6" flex is roughly the same as 4" rigid in most installations. At a MINIMUM you should have 8" rigid for your entire run if that is what your hood calls for (Many US hoods undersize this). If you have many bends then you should increase that to 10" to keep static pressure down.

A few very quick thoughts. Wide sweep smooth 45's & 90's will be much better than tighter radius or segmented. Two 30's would be better than two 45's and two 15's better than two 30's. An oversized end cap (eg, 8" to 10" reducer + 10" cap) will be better than a typical 8" cap. IOW, almost anything that can reduce the static pressure will help. 10" duct would help as well but might also result in too low of air velocity. MUA is Make Up Air. If you're sucking 600 CFM (with your setup about 100 CFM is more likely) out of your flat then you need to bring that same amount back in. In your case it is doubly important as too little MUA, as @kaseki mentioned, will cause your hood blower to have to work much harder against the vacuum that will be created in your house and so it's performance will go down even more. You need to do everything possible to help your hood do it's thing. You need to talk to a competent HVAC person.

"Finished" my installation today. I opened the wall and added in framing so that the mounting strip is held by 4 screws (the hood is held onto this strip by 2 screws). The roof vent (VentaHood, stainless steel I think) was rather easily installed as the roof has battens and spacing was wide enough for the hood to slip in between 2 battens and just replaced a tile. Straight up 2' from hood to ~65 degree elbow, 5' 10" to 2nd elbow (~85 degree) and straight out. All joints and seams metal taped and the ducting wrapped in insulation. Total vertical rise of ~6'. The roof vent has a flap held down by a magnet. Was worried after reading threads here that the distance and curves were too long and the air velocity and force would be too weak. 1 fan at half speed lifted up the flap half way and full speed all the way. Totally pegged with 2 fans running at full speed. The vent is a PITA to use as the flange is too short (just 1" so screwing in the elbow wasn't the easiest). Tomorrow, I will try making Cajun blackened chicken (Paul Prudhomme recipe) to see how well it works. One other note, the vent doesn't come with a pest screen. VentaHood advises against using one but birds and wasps are nesting pests where I am. I fabricated and will try a removable one and see how that works. Complication was that one of the lower screws for the warming shelf didn't hold (anchor failed) and the stainless steel flexes too much so I will have to open up the wall behind to add in a piece of framing. I will figure out a design for a duct cover (most likely will suspend it from the ceiling) and get that fabricated.

thank you. i was going to have a different fabricator make it for me, but in the end, they can't exactly match the stainless steel which is what was my concern from the beginning, but the HVAC sub had assured me that they could get very close when I initially decided to go this route. I just don't want to spend $1000+ for a "custom" stainless steel duct cover that wasn't going to look great in the end because the finish wasn't a really good match, even if they could install this without the screws showing. So I think I'm going to go with a custom hood cover made from wood which will get painted or plastered/sealed