I post my experience in case someone is going through the same journey to unlock the GPU. I have an ASUS ROG GL552JX. There is no much info online on this model, but it is a cousin of the ROG G751, which means some behaviours and BIOS settings are very similar (they share the codes for controlling the main fan too, which I control with Notebook Fan Controller, the fans let themselves to be set regardless of the BIOS version).
Anyways, I had installed the BIOS 209 from ASUS support webpage. Problem with this BIOS is that locks the turboboost for the CPU, lets you with 2,59 Ghz. So I downgraded to BIOS 202 (the earliest I could find). Using Intel Extreme Tuning Utility you can reach up to 3,8 Ghz (happy days). The vBIOS for the nVidia card is embbed in the main BIOS, so I lost my modded vBIOS installed (I got that from a forum or something two years ago, but I wasn´t able to find the thread, I think it was a thread on the ASUS ROG forum talking about the G751). So BIOS 202 throttles the GPU at 68ºC (and CPU unlocked), still got decent gaming but I wanted to get rid of that ceiling.
-To downgrade to an older BIOS I used WinFlash, if you try with AFUWIN is going to complain about BIOSes not having the same size. And once you're trying to flash with WinFlash you´ll see that does not allow to downgrade. You have to copy Winflash path, paste it in Command prompt followed by "/no date". There are videos on Youtube showing that.
-The vBios is a sector of the main one, and is 102,949 bytes (this is important cause there is only another section in the BIOS with a similar size). Did not manage to Find MMtool, so I blindly navigated through the hex folders to find the right sector (I used UEFI Tool to open the BIOS, with it you can edit, remove, extract, etc. but is very primitive as a GUI). Well, the right folder is the one that starts by "C5". C5D7EAAD-B218-482C-A909-E3B8CDB00E94. Inside that header is the magic.
-Used Maxwell BIOS tweaker 3.6, which was by far the easiest part of the process. Setting clocks, voltages and temp limits. I was moderate and just upgraded the memory clock, turboboost for the card and temp limit to 91 ºC (even 89ºC will work, remember you can always reflash or work with that limit with MSI Afterburner, i.e.).
-Went to UEFI Tool 0.21.4, removed the old "C5D7...." header and inserted the tweaked one. One nice check before saving the BIOS image is to double check that the thing you are removing and the new one have the same size, type and attributes.
-And last but not least, went for AFUWIN 5.05 and flashed the BIOS with the vBIOS addition. Restarted the PC and that was done.
The battery was on in any of the flashings. The laptop works fairly faster and it is also cooler. Roughly speaking, it passed from playing low settings to high settings with more FPS.