Were you comparing 316 with 316 wire in your pricing? That could account for a fairly large difference in pricing.
I would think going up slightly in wire size would be ok, but certainly not double as you want for some wires (lowers). The only good reason to upsize wire is if the original was wrong which is not very likely. Catalina made over 7000 30's and we would certainly know about it by now.
Wire should be tensioned to about 15% of the wire's breaking load. By going to heavier wire there will be a great deal more tension on the entire boat.
Wire (1 x 19) will stretch the same regardless of its diameter if tensioned to the same percentage of its breaking load. In other words 1/8" wire at 5% of its working load will stretch the same amount (about 1mm for each 2 meters) as 1/2" wire at 5% of its breaking load. The only way your old rigging stretched too much is if it was over tensioned.
You stated you have heavier chainplates, tangs, etc. But you have the same boat. All heavier wire will do when tensioned properly is bend the hull more. And there is nothing you can do about that.
Here is the best rigging guide I have seen, from Selden - they know a bit about rigging. It is a pdf you can download for free. http://www.riggingandsails.com/pdf/selden-tuning.pdf
The best 1 x 19 type 316 rigging wire available today is from Korea. Any good rigger can confirm this. Loos in the US is getting out of the rigging wire business and the riggers I have talked to state that the Korean wire is better than Loos wire as well.
And here is a fact from the Selden guide: