Others will address the question more clearly.
When you say mainline, do you mean the community kernel.org releases(kernel/git/torvalds/linux.git - Linux kernel source tree)? Or, SDK 2.0? What mainline are you referring to?
Not specific to this question, but see the following for useful information Why is QorIQ sdk base on yocto?
If the T2080 BSP changes have not been pushed to the mainline u-boot/kernel, then no it cannot be used.
If mainline means the open source community u-boot and kernel, the 4.8/4.9 mainline has not been validated by NXP and can introduce issues not found in SDK2.0 and SDK 2.0-1609 (SDK 2.0-1609 kernel release is 4.1.30; see SDK 2.0-1609 release notes below). As far as differences, completely different releases 4.1.30 vs. 4.8 and numerous differences between releases. See SDK 2.0-1609 release notes.
http://www.nxp.com/files/soft_dev_tools/doc/release_notes/QORIQ-SDK-2-0_RN.pdf?fasp=1&WT_TYPE=Releas...
