First, there is no Rampage for the z270 chipset.
Second, want you to take a look at this...
https://us.hardware.info/comparisontable/products/377817-388147-373875
the Strix E compared with two of the most expensive boards, Formula and Extreme(eATX). Under "Expansion Slots", if you check the " Available lanes for all PCI-Express x16 slots" section, they all have 20 lanes respectively and they are all capable of 2way SLI, no more, no less.
TLDR, there is nothing you can gain by upgrading the board as the Strix E already gives you the most this platform can give. O
Ofcourse the Formula and Extreme has reinforced Expansion slots, but that adds little value to performance.