Oh! for ride comfort going down to 45 series from 55 series and from 215 to 235 width, will sure be more bumpy.....
Yes, there'll be a drop of about 1.2cm of sidewall height/thickness, so it will be a bit stiffer. But IMHO will still be comfortable enough cause 235/45 still has about 10.6cm of sidewall, so still relatively thick enough, but again one's preference of comfort is usually different than others. I personally would not go below 10cm of sidewall.
Also agree with the weight of the tires even though might not be more difficult to choose/differentiate between them (I think max the difference is <1kg per tire). Note that even though you go with lighter rims, your 18' tire might end up be heavier than your stock 17', so there might be a slight trade off. The tire I found the lightest so far based on the data is the Yokohama Advan Sport, then Toyo T1 Sport.
In terms of FC, it should improve just a bit. The lighter you can go, the better the difference. If you can afford forged Japanese rims, it can be only 8-9kg in 18x8, so can save 5-6kg per rim, so total saving can be 20kg or more for the whole car! You should also notice improvement in braking and pick up.
