Ha, it's definitely a bit more complicated than this...Are there specific channels how the ECB decision will move the spread or just heightened volatility that can throw things in disarray?
My guesses would be:
QE programme change: both CTD's are about same issue size, with yields comfortably above depo rate, both non-benchmarks - so roughly same performance whatever the outcome?
Rate cut: same performance for cash bonds, but front contract outperforms (6 cents for a 25bps cut?) cus lower funding costs decrease the forward price (and thus the bund futures price), and with longer "holding" horizon the forward drop is larger for the back contract.
That said, I see a cut as very unlikely. Do nothing or taper the monthly volumes but extend the purchases past march 2017 seem more likely to me. Changing capital key seems slightly less likely, and no way they would lose face with a taper but without a sweetener.
For instance, if they allow purchases below the depo rate floor, it could mean a lot of shorter-dated bonds becoming available, which means they have to buy less long-dated paper. This could mean a steepening, which would make the back contract cheaper. On the other hand, it could also mean German repo becomes even more expensive, which may have the opposite effect. If they allow deviations from the capital key, it could mean a general cheapening of German bonds. If they decide to subtly suggest that quantity of purchases is an aspirational goal, rather than something to adhere to strictly, it could mean a broad selloff across all EUR sovereigns. If they relax the issue limits for all non-CAC bonds, bunds shouldn't be affected, broadly, but there could be some really funky basket effects. So you get the idea...
TBH, I have sorta given up trying to understand all the possible permutations. I think there are too many, the outcomes are hard to guess and the decisions are likely to be based on legal considerations, rather than economics.