You are over-exaggerating the SU's and -15's compared to future versions of the JFT. There is NO doubt that the SU-35 and F-15 are much superior platforms but the comparison of having 30 making SUCH a big difference and thinking 400 JFT's can be destroyed "before they can see the enemy" is VERY far fetched and untrue.
In fact, as of today, the range and modes of the JFT's radar is better than the -16's on paper. The range for sure is. But, I don't know the quality of the JFT"s radar and if its comparable to V-9 radars on Vipers. But on paper, JFT has more modes and range of the radar.
Plus the AESA would give a huge boost to the JFT. Don't take the JFT light. It isn't designed to fight with the SU-30 directly but it can send home any fighter in BVR as its the Radar, Avoinic and the Missile that do the BVR part. So the target can be an SU-30 or a Mig-25, the Missile doesn't go off the lock if it realizes its chasing after a SU-30. It still hits it!!
But generally speaking, YES, Pakistan needs the heavies and not 30. She needs around 80 of these. I'd think they'll start with 40's, get experience, build tactics and while the manufacturer comes up with upgrades, then they will get 20-40 more with more upgrades. Traditionally that's how PAF does things.
After 2020, you'll see about 25-30% of the PAF's fleet consisting of heavy, twin engine platforms. This whole financial struggle is till 2018 and then it ends VERY fast. But the dependence is that your current system, that has revived your economy, works the way it is.