pull development for provia, velvia

I am planning to embark on a minor photographic project, and I would like to tab other's experience of pulling possibilities of reversal films, particularly provia 100 and 400 and velvia 50. I am interested in enhancing their zonal range, and concrete suggestions about how much developing times can be reduced, with or without colour corection filter, would be extremely helpful for me, as I not only have to catch up with the characteristics of these films, but am facing labs here in Lahore I am not familiar with, and I would like to be able to give them as concrete instructions as possible.
The data sheets say Provia 400 can be pulled up to 1/2 stop (EI 280), Provia 100 and Velvia also 1/2 stop. I wonder whether more is possible, and what happens then?

I've pulled Velvia 50 with good results. It somewhat extends the dynamic range in both dark and light regions of the curve (reduces contrast). Colour saturation also decreases a little, which is usually okay with Velvia. Here are some results: Dead Link Removed (use your creativity with the file names. The numbers are ISO settings at camera.) Scanned with Nikon Coolscan V ED. Processed in Tetenal E6 3-bath kit.

I found that 20 ISO is a good exposure setting when the first developer time is reduced by 2 minutes. After scanning, I've matched the gamma of the following two to show the increased dynamic range in both dark (less noise from scanner) and light (more local contrast!) parts of image:

Dead Link Removed Exposed 50 ISO, normal development.
Dead Link Removed Exposed 20 ISO, -2 min first dev.

You can notice a difference in the bright wall. Even with more than doubled exposure, still there is more contrast in highlights when underdeveloped! So the charasteristic curve expands in both directions, giving a little less contrast in midtones but giving more definition to shadows and highlights. Or at least I thinks so based on this test .

I recommend this, if you have enough light to shoot at one stop lower speed and if you process your slides by yourself (you even save your valuable time by 2 minutes! .)

I've shot Provia 100F at ISO50 a few times, and I have been nothing but pleased with the outcome. It does drop contrast slightly, which can be helpful in harsher lighting conditions such as high sun. As long as the film lab knows to develop it at 50, it should be fine. Maybe even take a black permanent marker and write a "5" over the "10" on the film label (or cartridge) to drive home the point?