Looking at the way Russia inducted MiG-29s and SU-27s, there were considerable problems, it would be a long long story full of tales of downgrading radar requirements etc(later on, the MiG-31 with it's ~50 ton weight would carry a radar with the sort of specs the soviets wanted on the SU). Arguably, the 5th Gen jump is even bigger for Russia, mainly due to their avionics.
The US used to focus on independent components that came together in an aircraft, whereas the Russians tended to just focus on the end-product. Even in the 80's, the Pakistanis got customized F-16s. Russia is coming around, they're clearly focusing on avionics more, but they still have alot of problems - the T-50 does not have a proper engine, the SU-27 engine it flies with is not stealthy at all.
Time will tell whether the project gets delayed, logically, it seems very likely. Personally, I don't buy into this whole 5th gen bit. 5th gen aircraft are *not* immensely superior to 4th gen, the only major benefit 5th gen provides is reduced radar signature, and that only at the cost of severely compromising the aircraft's payload capacity(unless you go the T-50 route and build a gigantic aircraft which would naturally tend to have a bigger radar cross-section).
Other than the stealth(and the significantly reduced payload), all the avionics are already available in 4.5th gen aircraft, same generation AESA radars get installed, all these things can and are available in modern 4.5th gen aircraft. Smaller aircraft, even outdated ones like the MiG-21 "flying coffins" were hard to detect for the F-22, even though the -21 couldn't compete because it'd get shot when it got detected, and it's weaponry suite wouldn't be too modern.
So - I think PAF's decision remains justified. Focusing on getting modern avionics on our 4.5th gen fleet would enable it to compete perfectly in the short run, especially given that lately we're looking to acquire modern SAMs. In a defensive war like we're likely to have in any wartime scenario(India always crosses the international border, we never have), we'd do fairly well.