[Suicide Junkie]

I disagree with the idea of making research harder. If you do that you'll never get them before the enemy has phased shields and will never be able to exploit their special damage type.
IMHO PPB should be balanced to make them useful ONLY when you can exploit their special ability and become obsolete once phased shields come into play. A solution to acieve this should involve rebalancing not only PPBs but also SGs and PSGs (I recall complains about shields being too weak)

Have you considered making phased shields weaker than normal shields, but available at roughly the same tech level?

If phased weapons were set to be 60% of the strength of normal weapons, and you had the choice between a phased shield, and a normal shield that was 50% stronger, which would you choose?

You can keep both types of shields useful, while phased weapons become support rather than main-guns.