Supposing that leaks are true,
Could bulgarians do fast imperial into 2hs plus capped ram or onager in arena?
Probably. Fast Imperial into Onager is already possible, but not practical due to the upgrade time and resource cost taking so long to mass Onager that even a higher level AI has enough time to pressure. I think Capped Ram + full infantry upgrades should work far better than Onager. Siege Tower over Capped Ram might work better still. Siege Ram with less infantry upgrades might also be possible.
I mean, they receive the militia upgrades for free… and the blacksmith upgrades are also cheaper.