On a 5' wide table, 18" still leaves the standard 24" between deployment zones, so the only difference will be more deployment space and further for non-outflanking/deep-striking/etc. reserves to come on (such as Flyers). Not a huge issue, as it's just similar to the Hammer and Anvil deployment method.
A longer table (8' or whatever) with restricted deployment width (as you suggested) sounds fun for fast flanking manoeuvres, but will hamper units with the Outflank special rule. Again, possibly not a huge issue, depends on how many Outflankers you have in your group.
You'll also be able to play larger games

and I'm jealous of your available space!