30 might not be the right number, but 5 is way to low. At 5, there could be such a massive gap between the top 20 alone. Having 30 witnesses to vote for allows you select your top 20 and 10 backups. so 30 may have been based on a min framework??
Have you considered instead of changing the number of vote, increasing the number of top witnesses to say 50?
I also like an idea put forward previously about vote decaying and even rotation. we could even look at putting a life time value on a witnesses so each witness vote expires after 6 months