Where are you buying ProImage from? The reason I ask is that I saw a stunning Flickr posted by a guy based in Myanmar and Thailand and he uses ProImage 100, exclusively. When I tried to source some in the UK, I couldn't - but I found it in Thailand via E b a y and I have 12-15 rolls in my fridge.
I'm waiting for a warm holiday overseas or some bright sun in the UK to get the best out of the 100 ISO. I'm not tempted to push it as I want to get the benefit of its saturated colours and low grain.
To be honest, I suspect there'd already be a stampede if anyone produced a 400 ISO film with a colour pallette similar to Portra. You could try the Fuji 400 Pro (or the Fuji Sensia 400) as these are both excellent films. However, their colour rendering is a bit different to Portra.