Is it possible to "slim" down the lapels of a canvassed jacket? If so, how hard and at what price?
You can narrow lapels, but not on patterned suits as the line of the pattern is generally cut to follow the line of the lapel edge. Pinstripes are notoriously bad. Better to just buy a suit in the style you want.
Narrowing lapels can be done but most cheat and cut the edge and disregard the grain line of the cloth so it looks altered and askew. If the lapel buttonhole is cut thru, reducing at the edge is not possible. There are three ways to narrow the lapel without opening the outside edge but they have to know how and then do it skillfully.
Personally, I don't think restyling of suits is a good idea. Buy the style you want. If the alterations are not done well and you are not happy with the result, you lose in every way.
Most suits need some form of alteration, especially if you're purchasing an off-the-peg suit. You're very fornuate if you can get such suits to fit correctly and a keen eye can always spot one. If you're buying a suit for 500 you'd probably spend another 100 having it fitted correctly and there's not much point in doing that IMO because you can have a full-tailored suit for not much more.