K la... alot of people use the Hoya. I thought it was fine when I first used it too. That is, until I got my Nikon 6T's... then when I look through past photos, there is a distinct difference for me la. But still, alot of people are happy with it.
As for your lens setup now, I think ok la. 18-55 is a good range for walking about in the day. The 70-300 Sigma has a useful macro function too, that, though it doesn't get you 1:1, is still acceptable with larger subjects like dragonflies and butterflies if you can get close enough, right?
You want a zoom with greater range, meaning something like a 18-200 or 18-250 or 18-270 is it? I can't advise you on that though. I've thought about getting one of those, but somehow the urge isn't strong enough for me to get one yet. It'll be a good travel lens though, but image quality will be different from those with smaller focal range.
Actually, if u want, u can get Canon's 250D or 500D close-up filters, or Nikon's 6T if u can find them, or just get a Raynox, and attach it to your Sigma 70-300. I'm using a Nikon D300 with a Nikkor 75-300 lens with my 2 6T's infront of it. This setup, although super heavy (including flash) gets me acceptable macros... for now. One day, I'll get my hands on a 105, a 150, or even a 200mm micro. =)