I don't think the CCCP copied the F15 as such, what they did was create a new aircraft the MIG 29 and then developed other new aircraft from that such as the SU range of aircraft. The Russians are very good at building aerodynamic platforms but they are still some way behind the integrated systems the West enjoy (for the time being anyway...).
Now the next generation aircraft are nearing the completion of the test flying, that's when things will get interesting when the new stealth aircraft goes into production in large numbers to many customers we (the West) cannot sell our aircraft to for one reason or another.
Happy landing...