The TWT to GaN difference in input power without a linearizer is massive, as much as 4 times. So Bars will need 20+ kVA input while GaN would need about 5.5-6 kVA to get the same output.
With 2400 TRMs, you can use the same input power to get multiple times higher output. But it also means Virupaksha isn't meant to push out more power than what GaAs already does. Each TRM could be limited to 9 or 10 W at peak instead of the potential 25+ W, never mind hundreds of watts.
Furthermore, you need peak power for sustained high power operations for long durations, which is not necessarily a goal here. If MKI will continue using the old generator, then its more efficient for sustained operations at 50% power, so the best you can use is 60 kVA in total with 2 generators. If 20-30 kVA is used by the radar, other electronics need to become more efficient to compensate when the radar is at peak. Alternatively GaN can provide much greater performance than Bars when operating at significantly lower power without having to take away power from other electronics.
So there are ways to manage power without having to upgrade the generator. Although it's desirable to see it pushed to 75 kVA on the MLU 'cause you never know what's coming just a few years later.
Virupaksha being a drop fit is great news, it means even non-MLU jets can still get a radar upgrade at the bare minimum, not just the EW suite.