Great work Gafferjack. Using your data, I have determined the functional relationship between base proficiency, weight modified proficiency, and armor weight:
armor weight modified proficiency = base proficiency * (1 - 0.0082 * effective armor weight)and
effective armor weight = 3*head armor weight + body armor weight + leg armor weight + 2*hand armor weight - 7.7I think we can safely assume that if the second equation would result in a negative value for effective armor weight, it would return an effective armor weight of 0 instead. The following plot was generated using your 11 data points plus a 0 effective armor weight point. This plot shows what fraction of your base proficiency your weight modified proficiency would be for a given effective armor weight.
These equations show that the reduction in proficiency due to armor weight is now multiplicative with base proficiency and that you can wear 7.7 effective weight of armor before your proficiency is affected. The equation for effective armor weight appears to be very much like the previous version with head and hand armors effectively weighing more than the listed values.