Oh the irony :-) The older hashing algorithm distributes data unevenly across buckets in the hash table, so some buckets have far fewer/greater items in them than others, however it is distributing individual items better. At least, with certain combinations of data. With some combinations of data, it leaves many buckets empty.
The fnv1a algorithm distributes items more evenly across all buckets, however, due to its lack of avalanching, more similar items are being stored in the same buckets. This means that when a hash matches a bucket and it has to be searched, even though there are fewer items in it (because they're more evenly distributed across buckets), the string comparison call takes longer as it is matching more characters per item because they are more similar, even though there are fewer string comparisons.
All I can say is that you have put a lot of work into analysing the impact of this proposed change - and we (the mIRC users) are lucky to have someone as dedicated and diligent maintaining this tool.