First, they do not use same BIOS. Second, be VERY careful when you see any BIOS Size error, do not flash or you can brick the board in a hurry if it allows a flash by you somehow forcing that to happen.
Z97I-Plus, easiest way to flash this one with mod BIOS is via USB Flashback. Smaller cheaper USB better (128MB-2GB), put mod BIOS on root of USB (FAT32 formatted) and BIOS must have SB Flashback name >> Z97IPLUS.CAP
If you have a rom or bin file now, you cannot simply rename this to .cap, you must put BIOS back into capsule via hex editor, then rename. If you are not sure how all that is done, upload your mod BIOS for this board and I will make the file for you.
Here's how USB Flashback works
https://event.asus.com/2012/mb/USB_BIOS_...ack_GUIDE/
https://www.asus.com/us/support/FAQ/1013998/
https://rog.asus.com/technology/republic...flashback/
H97I-Plus, this does not have USB Flashback, best method I suggest for this board is to dump BIOS region via FPT, unlock BIOS lock via grub using setup_var, then flashback mod BIOS region via FPT. Since I've mentioned FPT and we're talking Asus in general, NEVER Flash stock .CAP BIOS in any way with FPT. BIOS region can be removed from stock BIOS and flashed via FPT, but that is not suggested either, you will loose serial, UUID etc. Only dump BIOS region with FPT and then flash that back after you've modified it. SO, in other words, throw away any BIOS you have modified for this model now, so you're not tempted to use it with FPT.
Here's how to do all the above mentioned
Get BIOS region via FPT and flash via FPT - Check BIOS main page and see if ME FW version is shown, if not then download
HWINFO64 and on the large window on left side, expand motherboard and find ME area, inside that get the ME Firmware version.
Once you have that, go to this thread and in the section "C" download the matching ME System Tools Package (ie if ME FW version = 10.x get V10 package, if 9.0-9.1 get V9.1 package, if 9.5 or above get V9.5 package etc)
https://www.win-raid.com/t596f39-Intel-Management-Engine-Drivers-Firmware-amp-System-Tools.html
Once downloaded, inside you will find Flash Programming Tool folder, and inside that a Windows or Win/Win32 folder. Select that Win folder, hold shift and press right click, choose open command window here (Not power shell).
At the command prompt type the following command and send me the created file to modify >> FPTw.exe -bios -d biosreg.bin
Right after you do that, try to write back the BIOS Region dump and see if you get any error, if you do show me image of the command entered and the error give >> FPTw.exe -bios -f biosreg.bin
If you are stuck on Win10 and cannot easily get command prompt, and method I mentioned above does not work for you, here is some links that should help
Or, copy all contents from the Flash Programming Tool \ DOS folder to the root of a USB Bootable disk and do the dump from DOS (FPT.exe -bios -d biosreg.bin)
https://www.windowscentral.com/how-add-command-prompt-power-user-menu-windows-10-creators-update
https://www.windowscentral.com/add-open-command-window-here-back-context-menu-windows-10
https://www.laptopmag.com/articles/open-windows-10-command-prompt-administrator-privileges
How to unlock BIOS Lock so you can flash mod BIOS, you can start at step 6, I've done rest for you below
https://www.win-raid.com/t3908f16-GUIDE-Grub-Setup-Var-Guide-Fix-Intel-FPT-Error-BIOS-Lock-Bit-Set-HSFS-W-Asus-or-Other-Mod-BIOS.html
BIOS Lock variable to unlock at grub >>
0x8D
Rename your .efi file to >>
Shellx64.efi
So at grub prompt you'll type the following >> setup_var 0x8D 0x0
Then reboot to windows and then flash in your newly modified biosreg.bin file