I just did two applications, a week apart. My curl pattern didn't decrease at all, but I'm sure I haven't done enough to see if it will or not. My hair was a lot curlier after using half henna and half amla. I wouldn't recommend the 1:1 ratio though, it had a bit of a drying effect. I toned it down on the next application to a 2:1 ratio and my hair feels amazing!
I think there may just be anecdotal evidence of amla helping keep your curl pattern when using henna.
I was wondering too - how the heck do you apply just to your roots? Will there be an overlap that is darker and look weird?