Interesting, as the actual "put code in the mclk and sclk" type stuff in sysfs does NOT actually change the clocks when I tried it on my 4 x R9 290 rig or my mixed R9 280x/HD7850/HD7750 rig.
Did you check /sys/kernel/debug/dri/0/amdgpu_pm_info to see if the clocks actually CHANGED? (change the 0 for the specific card being checked of course).
Might also be a "R9 3xx" vs "R9 2xx" difference, or a GCN 3 vs older GCN versions difference, if it DOES actually work - I only have R9 290/280x + HD 7870/7850/7750 cards to test on while GCN 3 wasn't introduced 'till the R9 285 (Tonga) card.
an old thread, but a few things for anyone that runs across it that still cares about using old cards: at least GCN 1 and 2 don't work with the amdgpu way of overclocking, AND some of the cards have "quirks" coded into the driver that sets a max speed, so even a vbios mod to, say, take the memory of a 7850 and run it at 1500mhz won't work because it's coded into the driver to not allow it over 1200mhz, Core clock was also maxed out at one point but I think that got lifted on the 7850. It's possible to mod that part of the driver even with the amdgpu-pro one.. once that's edited, you can change clocks and voltages for cards in the vbios. hex editing and VBE7 are the things to do that for pitcairn (7850, 7870, r7 265 r9 270/270x and r7 370.) tahiti (7950, 7970, 7990, 7870 XT, 280, 280X). I'd expect it to work on 7750 and 7770 too, but not 7790, as 7790 is actually GCN 2 like the 290(X) and 390(X) are. Not sure if Hawaii Bios Editor can handle the 7790, but otherwise that's the tool for those. GCN 3 cards, like the R9 285 and 380.. not sure if overclocking works with the amdgpu way, and I'm unsure what vbios editing tools work with them.
edit: as for the 7870 XT, which is different from the 7870, it's got a Tahiti core stripped down to a 256bit bus like Pitcairn but has more compute units and better FP64 performance on those units than Pitcairn has.