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so what i understand is, even if i prioritize traffic from Branch Towards HQ Out,
when someone is downloading some huge file in HQ, there is nothing i can do.
If the huge file is coming from the Internet, then yes, you have a QoS problem. If you mean from a branch, you only have a problem with traffic from multiple branches (especially if their aggregate oversubscribes the HQ link).
what about if i prioritize from HQ towards Branch, will it be beneficial? my point is when traffic will leave HQ upload to reach Branch in download direction IN, it would have congested Branch ingress interface already and then it will open packet to find out its priority etc, so i dont think it will be beneficial in that case.
It depends on the bandwidth on the branch. If, for example, the branch only had 10 Mbps, your HQ sending at 25 Mbps wouldn't even queue at HQ, but would congest on the link to the branch. The usual way to address this issue, is have each tunnel have a shaper, with prioritization QoS, that corresponds to the branch's bandwidth. Some Cisco devices will support that and physical interface QoS too. Unsure any, though, will support both a tunnel and physical interface shaper or a hierarchal physical interface policy that supports both a parent and child shaper.
If one device cannot do the two level shaping management, you could consider next upstream HQ LAN device (if capable) shape and prioritize for 30 Mbps, and use the router to further shape for each branch. I would recommend such an effort, as QoS can be great for congestion management, but again, if you're sharing the link for general Internet access, this creates ingress traffic flows you have little bandwidth management control over.
