Keep in mind that the Sigma 18-35mm is a very large lens. But it is said to be very good (I haven't had one). I you don't mind the size, I'd say go for it!
The Pentax 18-270mm will give you a bit more reach like Norm said, but I'm not sure it will give you a huge step in quality. Probably an improvement in color for those who prefer the Pentax colors like a lot of us do
The Irix 15mm would be more of a specialty lens, I think, since everything about it is manual. Is there anything about your SIgma 10-20mm that you are unhappy with? I've seen some beautiful shots taken with it, so I think it is quite capable. I'm not into super-wides, otherwise that's one lens I would probably consider.
How about complimenting your superzoom with some primes that you can use when you want to shoot in lower light or have better quality pictures? And for macro as well - perhaps the DFA 100mm f/2.8 Macro would be a good place to start? And I find having a fast all purpose prime like the Sigma 30mm f/1.4 Art to be invaluable to me - very fun to use despite being quite heavy for a prime lens, but not nearly as heavy as the full frame fast lenses from Sigma... and probably light compared to that SIgma 18-35!