there is no XP booster, GW1 caps out at lvl 20 which is reached after mere hours of game time for story characters and thus completely DEFEATS the point of having xp booster, while pvp character is AUTOMATICALLY maxed out anyway level wise and item potential wise at the start so again have no use for XP booster, but you can buy more bank space
no advantage means, none of the item you can buy on the store will give you a direct advantage against another player that he can't acquire without the shop.
you can argue the bank slots is technically an advantage indeed, which i'd partially agree but to be honest in GW1 that's more the problem that they didn't have a robust auction system, something they are planning to remedy this time around.
GW1 was solid PvP wise, and it has been so given their significant PvP content... they have a fairly CLEAR line of what constitutes an advantage against other player and what is not. So unless you have something to substantiate concern on the PvP side i'm not quite sure where your worry came from.