@Psions Consider that to "break" a walled T1 PD (260 mass), you only need a single T1 artillery (36-54 pass) and about 30-40 seconds. This means T1 siege is effective at 20% mass cost investment.
Now consider a T2 PD + a TMD. For Cybran this costs 480 + 280 = 760. To maintain a similar siege effectiveness, a single MML (200 mass) should beat this in ~30-40 seconds. It does not.
In a test game, I build a Cybran T2 shield + 2 T2 PD + 2 TMD. This costs 1480. In a gametest, I tested 5 Seraphim MML against this, total 1000 mass, and it took them one minute to destroy this setup. (Actually, the MML couldnt destroy the last T2 PD because they kept missing due to a bug.) This indicates that a much larger investment of MML (proportional to defenses) is required to crack a T2 firebase, and it takes much longer to do it. If MMLs are made more effective, they will defeat T2 firebases before being destroyed my T2 artillery themselves.