Depends on the quality, you said that you got PR7 and 8 backlinks, maybe yes. I think only 1 or 2 so continue link building with high PR sites
You will not get a PR6 or higher from a single PR7 link with 15 outbound links. I am 100% positive of this. If the page only had 1 obl, you could expect a PR6. With 15 obl, the link will only pass 1/15th of the PR to the site. I have had PR7 links, with less obl, and the sites never went higher than a PR5. Check the wayback for the 15 sites that had links before the last PR update. You'll likely see several that didn't get a PR6 - and they probably weren't all starting from zero.
If you have a good internal linking structure a single PR7 or PR8 link with less then 10 outbounds could yield a PR4-5 providing the link is set up 2 months before the update! That is just a guess by the way but i would imagine that it is not to far from the truth!