Markus, don't get me wrong, I'd use a closed deck block in a flash for a higher power build for exactly the reasons you are. Much as I feel the production quality of the open deck block is higher, it's still an open deck which is a pain if you want to reliably run at high boost pressures (ie greater than 1.5 bar) IMHO
I'd be curious to see what can be done with a semi closed deck block, but know that the EJ257 block isn't up to the job in it's standard form (the liners are far too thin)