Slowman wrote:
let's look how the frames compare:
old slice 51: stack = 49.4, reach = 37.1
new slice 48: stack = 48.7, reach = 37.5
if it's the same front end on each bike, the length is within 4mm, the height 7mm. if you slapped 7mm more of spacers under the front end, that raises the new 48 up to match the old 51. but that is going to pull the bars back about 2mm, so now you have a front end that is 6mm closer to you. you have to decide if that makes a difference; if you want to get another, longer stem. not a big deal.
i don't know about the headset top cap, but those are made to be changed. also, when i got my supersix evo it came with what looked like a tall top cap but when i took it off there was a mini top cap, only 5mm or so, underneath. pretty nifty.
If I can take off the top cap the new slice 51 (stack 50.7, reach 39) ends up being similar to the New P3 (50.6, 39.7). Based on your measurements, the old slice 51 and the new slice 48 are pretty well the same, so I would need to decide which end of the spectrum I want to go with (less spacers, shorter stem on the 51, more spacers, longer stem on the 48).