Yes, portals will be unnecessary in Corona 6 (and in fact options to add them will be gone). Sampling from an HDRI is exactly what is improved with the new Env Sampler.
As to the noise, this is exactly what denoising is for :) Some places in a scene will take a long time to clean up, and that's how and why the denoiser can reduce render times by up to 70%. Highly recommended in this case.
As for noise limit, yes, it's non-linear, it will take significantly longer to reach 2% than 3%, and so on. But, denoising is the planned solution for that, you should be fine hitting 3% and denoising will take care of the rest - you just need to get it "clean enough" and let 0.5 to 0.6 denoising take it from there :)