Okay so we are talking about armpad x/y. Not frame stack/reach.
Is that pad reach to the front edge of the pad or the middle of the pad? If it's a Retul fit and they used the Zin measuring device, that will be to the front of the pad. The Felt Excel fit calculator uses the measurement to the middle of the pad.
One last question - what's your saddle setback? If the saddle can come forward, your armpad reach can come forward as well. In the case of the measurements you provided, we'd almost certainly come forward if the bike allows.
Using the Felt fit calculator and assuming the measurements you provided are to the middle of the armpad, 54 is definitely the right size with 20-25 mm of spacers under each extension. However you will not need a shorter stem unless you change the aerobars. The Felt bars have armpads attached to the extensions and can be moved fore-aft significantly.
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