I find the F-15C itself is vastly inferior to the Su27 ingame. For me, the only two advantages it has are TWS and the RWR giving a better general picture.
The RWR part is obviously up for discussion. I personally dislike it because it makes me over confident and I prefer the signal strength precision the SPO15 gives me as well as the relative altitude.
In the performance department, I don't like how unresponsive the F-15 is at low speed and how easy it is to bleed it even without external tanks. On the topic of fuel, I don't like having to get rid of most of my fuel before a fight (I find the Su27 to be acceptable even at max fuel, mostly because it's still maneuverable at low speeds). The F-15 is an energy fighter whereas the Su27 is a turn fighter, but I think the HMS + IRST + R73 combo beats the F-15's AIM9M by far.
Regarding weapon systems, the Su27 has the advantage with the IRST (which I am aware is not as effective IRL) as well as the ET. Then there's the R73 which is vastly superior to the AIM9M as said above. Lastly, the Su27 has two extra pylons.
The radars are similar although I prefer the way information is presented in the Flanker, but that's personal preference. TWS mode is useful in the F-15 but I'd take the IRST over it - TWS having the range advantage but setting off RWRs.
But... the F-15C can carry the AIM-120B/C and that makes it the king of BVR. However, when you strip the AMRAAM off the Eagle, it gets dominated by Flankers (keeps happening in 104th's operation bison).
Bottom line, I don't think the F-15C itself is a great plane. There's nothing about it that makes me go "wow, I'd take this to combat any day". I think the AIM-120C is a great missile that makes the F-15 look like a great plane but when the F-15 doesn't carry it, the plane is mediocre. Unfortunately the AMRAAM is still nerfed in DCS.