Well,you may or may not be planning a wasted trip.Hell,why not go to the western end of the lake at that time.
Fishing for salmon at that time,out of the ports of Point Breeze,Olcott,Wilson,and the Niagara bar is usually very good.
I fish Olcott every year the first and last weeks of may,and the fishing is great.Last year from may 21 to 26 was phenomenal.
We had one day with 30+ kings on,and boated twenty five of them,all in the mid to high teens,and one slob at 27 lbs. Big for a spring fish.
As for Mexico,we used to fish it every spring,and had some very good late may Brown trout trips.It depends on the weather,but you should be able to do ok.check out
www.lakeontariounited.com there's a lot of good info,and helpful guys on that site.