Even though your highest GFX divider in UEFI is only 900Mhz you can still raise it from that using APU Frequency, also, if you have a couple of gigs of ram to spare you can raise GPU performance by going into Advanced Screen>North Bridge Configuration>Shared Memory and raise it to 2G.
A modest OC in Windows using OCTweaker should solve your problems. You will be trying to produce four realistic operating frequencies, CPU, RAM, GPU and NB, from the following input values:
Pll: otherwise known as CPU Ratio.
Apu: a MHz value commonly known as Bclk, or Processor Frequency or APU Frequency.
Mem: a memory divider, eg. one of 800/1066/1333/1600/1866/2133/2400MHz.
Gfx: a set of graphics clock dividers derived from the processor and listed in BIOS in GFX Engine Frequency.
Nbg: a MHz value of the Northbridge frequency divider.
Using the following equations: Cpu=Apu*Pll; Ram=Apu*Mem/100; Gpu=Apu*Gfx/100; NB=Apu*Nbg/100;
So by raising Apu Frequency to 105MHz using a 900MHz Gfx divider you would get Gpu Frequency to 105 x 900 / 100 = 945MHz. How much further you can raise Apu Frequency on your system is a matter for experiment, but this may depend on whether you have Sata set to AHCI or IDE mode, leaving you with a compromise to make between faster drives or faster clocks. However, to get near your target 1028MHz you'd need to raise Apu Frequency to 114Mhz (114 x 900 / 100 = 1026) .
As a guide to NB voltage, on the Asus A88X-Pro running an A10-7870K raising the GPU dividers through 866/975/1050MHz automatically raises VDDNB through 1.225/1.25/1.3375V.
PS. In raising GPU to 1026MHz using a 114Mhz APU Frequency, of course, you will also be raising CPU, RAM and NB. At default values, assuming a 2133MHz memory divider and NB at 1600MHz, this would result in CPU=114*37=4218MHz; RAM=114*2133/100=2432MHz; and NB=114*1600/100=1824MHz - which should all be possible on your system without further voltage adjustments apart from VDDNB.
Edited by Himo5 - 2/10/16 at 6:37am