There really aren't any downsides I can think of off hand. How much range increase you get depends on how well you do it. That's like asking how much range you should be getting after you install x y and z parts. It all depends on how well you do it and how well you can fine tune it later. Read through the "diy one piece barrel spacer and LRB guide" in the general section. Read the comments too. That should tell you what you are looking for.
How easy it will be to do an LRB in a suppressor will depend on what's inside. If it is just a tube inside the suppressor like the outer barrel, it shouldn't be too hard. If it is a bunch of foam or a hollow space, it will be a little harder. Like I said, look through the LRB guide in the general section (it's stickied) and there are different methods people use of doing the LRB.
One more note on the LRB: if you do it, put the rear spacer at the rear 1/3 of the barrel and the front spacer at the far front. It's better to have a gradual bend rather than a sharp turn. And don't do the LRB if you have a bull or a steel barrel. After a little while they will hold their bend, unlike the thinner normal brass barrels.
