(I guess that's a sort of trivial answer? Feel free to ask follow-up questions if you're genuinely curious about it.)
Usually I see startups handling this by having a "call us" pricing mode, which I have always assumed meant "We want to establish a proper business partnership with you that goes beyond being just a user"
We could certainly adapt Webscript to be a better product for bigger workloads, but that would be a lot of work on our side, and that product might end up just being "Heroku but with Lua," which is probably a bad product. :-)
I also doubt that your experience applies here in terms of those heavy users wanting 2x the use for 4x the price. At 4x the price, Webscript would cost close to $20 per month, and we would start to compete price-wise with shared web hosting or VM hosting solutions. I'm not sure there are really users who want something like Webscript for that price.