Don't forget the shock length and ride height are two different things, though longer shocks very likely will indeed yield a taller ride height with similar springs set up the same.
I see quite a few comments about all sorts of chassis changes, wheel and tire widths etc, that end up with something like "I did that and didn't have a problem". I do not doubt the sincerity or integrity of these statements. I have on occasion looked at and talked about bikes with their owners who have made alterations that I thought questionable and in the main they have seemed pleased enough with their machines. However after riding around with some of these guys, it seems to me they simply aren't inclined to lean all that hard on their bikes, and as a result the changes they have made don't stand out in any sort of alarming way.
But that does not mean that those modifications have not adversely impacted their bikes. Motorcycle chassis dynamics are considerably more complicated that it would seem from such a simple looking machine. That said, making basic changes like longer shocks (that provide the corresponding taller ride height)(or shorter ones), or lowering the triples on the fork tubes (or raising them) will without question have a predictable affect on the handling. Whether that effect will actually be dangerous may depend on how you ride, and what the bike was like to begin with. Raising the back steepens the rake and reduces trail, making an already inherently unstable assembly even less stable (talking about typical standard geometry here, not a chopper with extended forks obviously, though the science still applies). Take a bike that is generally good handling and well balanced and add 2 inches to the back with no other changes and you will cause a very profound and noticeable change to its manners. Exactly as XB has promoted, and if you are in the habit of pushing the bike to its limits, just as dangerous, or at the least you will realize how much you have compromised your bike. If you stay close to the speed limit and not planning on dragging any hardware in the turns then maybe the trade off is worth it.
As far as the 16" to 18" wheel change goes, raising the back is raising the back. It does not matter whether it is shocks, frame or wheel. However, it is the end tire outside diameter that counts, and your 18" tire may be similar to that fat 16".
And keep this in mind about your bikes "stance" We all care to varying degrees about how our bikes look, but I think it is a safe bet that it is high on most peoples lists. Change how your bike looks with the bodywork, not the suspension height. The suspension has a job to do. Get it sorted out so you like how your bike drives. After that, you can make the bike look any way you want by altering the tank and seat and still have a bike that is good to ride.
