No. Firmware does not change your GPON sync rate (limited by your ISP plan). However, it reduces latency and jitter, making your connection feel faster.
ISPs can still manage the router, but top firmware limits their access to read-only (unless you grant them). You can now disable remote management permanently via the hidden menu:
http://192.168.1.1/cgi-bin/remoteinfo.asp hg8345r firmware top
Solution: You bricked it. But don’t panic. it reduces latency and jitter