Stian,
I am unsure what model XS you have. If it is 77 or older, it definitely did not come with a top out spring. This creates a loud "clunk" that is both felt and heard under hard accelleration, when the fork extends fully. This is definitely an issue for me with my 77 XS.
I can agree that a stock XS with 320 mm shocks and a stock fork looks a bit "raked out",
but the cure for both looks and function is definitely not any simple 2" lowering spacer.
Longer, good quality rear shocks will help quite a bit, I'm sure Grizld agrees, maybe 15 to 25 mm longer should be fine. With shocks, as most other things, you get what you pay for, and often less than that. Get a set of longer shocks with spring rate for your bike, your weight and riding style. I have had Hagon, and did not like them at all, in fact I would like to sell them cheap. Some cheaper "racing style" shocks with external reservoir work really poorly, for example the red Marzocchis from the 80s, and Paioli from the same era. I do not believe any current Chinese/Taiwanese shocks are much better.... Now I have basic emulsion type Ohlins on mine. Wilbers, WP, Race Tech, Nitro, etc make good shocks, and can set them up for you. The cheapest shock I would even consider is Koni/Ikon.....
Now regarding the fork.The XS fork has 150 mm travel, while modern (sport) bikes have mostly between 120 and 130 mm. So slight lowering (20 to 30 mm) may be ok, if spring rate and fork damping is set correctly. Mostly this would mean a fairly expensive order from Race Tech, new LINEAR springs, and "Gold Valve Emulators". Of course new seals and oil of the corrext viscosity as well. Then play around with spring preload, oil viscosity, oil level and the compression damping adjustment on the Emulator.
And, like I have posted before, any lowering spacer should incorporate a top out spring!
Maybe a better option is to do as Richard Pollock used to recommend, 38 mm forks from a Yamaha FZR400 or FZR600, and TT500 triples bored out to 38 mm. This setup is what I dream of, it is a lot lighter than stock XS, as well as stiffer. It may even be possible to use FZR calipers, and do away with any adapter plates if modern calipers are on the menu.