I've had a complicated relationship with the 18-55mm over the years.
When I first started moving from slide film to digital I utterly hated the 18-55 because it had a rendering style so far away from what I was used to with film era lenses, and I threw it in the bin. Literally. Instead I shot with Takumars and loved the film-like look they gave me with a CCD sensor.
Somewhere along the line I decided some weather resistance would be useful, so I tried the 18-55mm WR and hated that too. I ended up sandpapering the front element to try to give it more of a vintage lens rendering style -- which actually helped to some extent. Then I tried the 16-45mm/4.0 and hated it even more (lot of hating in this post; sorry about that).
But over time I started getting tired of lugging around a bag full of heavy metal Takumars, no matter how much I loved their rendering style, and a year or two back I decided to give the 18-55mm another try. I put a lot of work into custom .dcp profiling to bring the colour rendering closer to what I wanted, and I enjoyed the light weight and the convenience of the zoom. Today I do almost all my photography with the 18-55mm, and it suits my present day needs perfectly well. Like you, I've considered the 16-50mm but looking at sample photos online hasn't convinced me that I'd get enough of an improvement for the extra size and weight and cost. Also like you, I've considered the Tamron and Sigma alternatives but again the sample photos that I've looked at haven't made me want to switch.
The one lens that I really would consider switching to is the DA 20-40mm, because whenever I look at shots taken with that one I think it would really suit me.
So anyway, sorry about the long post. The takeaway message is, no, you don't really need to upgrade from the 18-55mm. Not unless you need something wider or longer or faster. But if there's another lens that you really think will suit your photographic tastes better then yes, absolutely, go ahead and make a change.