I would suggest that you compare the potential gain if you...

I would suggest that you compare the potential gain if you invest the full $15k now vs. the potential gain if you invest in the SPP and get the full $15k and if it is scaled back and you only get $10k or $5k.

Then compare which potential gain is largest (and most likely) and go with that option.

You could also consider investing $5k or $10k now and subscribing $5k or $10k in the SPP as well to get the best of both worlds if you expect it to be scaled back

Please remember this is just my personal opinion and I am not a financial professional so DYOR!