The A15 piston used a lower deck height than the A14 because, with the same rod as the A14, the A15 crank pushed the wrist pin higher up the bore, so a lower crown was needed to prevent the piston from poping out the top.
In an A14 it will fall short of the top of the block by about 2.5mm.
Addidionally, because the piston in an A15 displaced an additional 22cc of air in it's journey from the bottom to the top, & it was trying to squeeze it into the same size combustion chamber as the a14, it became necessary to make a deeper bowl in the piston crown to bring the compression ratio to about the same as the previous engines.

With A15 pistons you will drop the compression by a significant amount & even the squish band will be non functioning with the piston falling short by 2.5mm so this can't be good.
Even flat top A15 pistons wont save it.

The beauty of the later intake manifolds is the fact that they are water heated so make sure that this feature is fully operational even if you enlarge the holes under the ports fractionally with a drill.

You know, drivers of more powerfull cars may pass you easily on the freeway, but the smile you get when you top up the tank for pocket change while they re-mortgage the house to fill up can make it worthwhile sometimes. I know it does for me, particularly when fuel prices hit a record high recently. I was laughing all the way to the bank.

P.S. What most people never realise is that the most efficient engine is a constant speed engine as all of the parameters can be tailored to this specific operating environment for maximum benefit.
This is why large reciprocating piston aircraft engines are constant speed engines using the prop pitch like a sliding ratio trans to handle speed & load.

This is why professional 'economy drivers' try to keep their vehicle speed, & at least the engine speed, within a narrow rpm band wherever possible. Once this optimum engine speed is determined, keeping it there is an edge you can use.