In the current version of Area51 the 5 question limit should be maintained. While I have often wanted to write more than 5 questions on a proposal, the limit makes sense.
The question writing goal of Area51 is two fold, define the site AND validate there a good likelihood of having sufficient content to sustain a site. Optimal each question writer would write 4 good (10+) questions and 1 bad (0 or negative) question to help define the site. Best case with a 5 question limit this is the questions of 10 people (4 x 10 = 40 Questions with 10+ votes).
I am personally a fairly active question writer. I have written 112 of the 1460 questions on Pets.se and 94 of the 1,688 questions on Space.se On these two sites that are doing well I am contributing in the 5-10% of the question volume.
On an Area51 proposal 5 questions is 12.5% of the total 'required' questions, but if you knock it down to the 4 good and 1 bad (where bad is out of scope), than you have 10% of votes on a proposal, being from one person. Which for a beta site is equals one of your 2 or 3 highest question posters.
There are a couple of proposals that I could easily write 80 questions on, half of which could probably capture 10+ votes on an area51 proposal. If this were to occur, and the site went live, where would the other 90% of the questions that are needed to make a site prosper come from? Maybe the will come in anyhow, but if you can't get 40 top quality questions out of your prospective members with a 5 question limit, you have a very thin chance of having a successful launch.
The limit should stay at 5, in no case should it be raised for quality question writers. I could make a good argument for lowering the allowed questions to 3 per user, but given the current state of Area51 I don't think that is a good idea.