RX 580 HEVC support on Catalina 10.15.4 ?

Tried following scenarios, when rendering HEVC in Compressor (4.4.6), none of them fully utilizes RX 580 and causes CPU to maxed out. When doing H264, only #2 fully utilizes RX 580.

#1. iMac15,1 with headless IGPU and RX 580
=> H264 and HEVC both don't fully utilize RX 580

iMac15,1 HEVC

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iMac15,1 H264

#2. iMac15,1 with headless IGPU and RX 580 and boot-args: shikigva=40 shiki-id=Mac-7BA5B2D9E42DDD94
=> To my surprise, this gives me the best result for H264 as CPU not maxed out, as you can see the CPU power stays low. HEVC still maxes out CPU.

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iMac15,1 HEVC with shikigva=40

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iMac15,1 H264 with shikigva=40

#3. iMacPro1,1 with RX 580 only and IGPU disabled on bios, tried with or without shikigva=40 shiki-id=Mac-7BA5B2D9E42DDD94
=> H264 and HEVC are about the same as iMac15,1 as CPU maxed out

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iMacPro1,1 HEVC

H264 is about the same as HEVC on iMacPro1,1

I know Haswell doesn't have HEVC support, so I hope RX 580 can be utilized. Since Apple removed Polaris HEVC support on 10.15.2, does it mean RX 580 still lack HEVC support on 10.15.4? Are there any way to make HEVC working with RX 580 on 10.15.4? If not, does upgrading to 5700 XT solve this HEVC issue? Any comments would be much appreciated!

My setup:
Catalina 10.15.4 with OpenCore 0.5.8 (followed vanilla guide), Lilu 1.4.4, WEG 1.3.9
SMBios: iMac15,1
CPU: Haswell i7-4790K 4.0GHz
Motherboard: Gigabyte GA-Z97X-Gaming 5
GPU: Sapphire Pulse RX 580, HD4600 (headless IGPU)
Memory: Corsair 16GB 1600MHz DDR3
SSD: Samsung 850 Pro 256GB