Hi Popsha, first of all welcome to BCM Touring. Please use the
Introduction Forum to tell us a little about your self

.
Coming back to your query, due to the FKR the height would increase by half an inch or so. But the ride doesn?t gets too harsh due to it. But the suspension would be doing overtime and its life will reduce a little due to it and so will the life of the chain set.
As for putting gas shocks, well I wouldn?t recommend them. Over time they develop leaks especially if you are into offroading and touring.
However if you wish to put them on your 180, then you would have to change your 180 classics swing arm to either DTSI v1 or v2 swing arm. Which means you would have to change a lot of things; chain set, brake rod, bush etc. last I heard the whole procedure would cost some where around 3-4k.
As for the grip of FKR on the tarmac, well this is the reason I love them so much. They perform just as well onroad as they do offroad! I know it is hard to believe, but due to the soft compound used in their construction they provide excellent grip on the tarmac, especially while cornering!
Oh yeah one bad thing, you will have to remove the chain cover since 120 is an oversize for 180?s. Stil I would say go for FKR, they wont let you down!