So I wanted to investigate - whether the 'original' firmware is able to run full speed - but it turned out to be a major problem to get back to original DLink firmware.
I've tried even possibly some 'stupid' moves 'aka 'mtd write dir.bin kernel' - which was likely the only variant somehow switching to the original firmware - however producing gazzialion of 'jffs' errors - as likely same partition was badly formated/had errors/(posssibly rootfs).
So I'd likely need here some help.
I was able to get back to OpenWrt which works normally (expept for 100Mbps WAN port 
)
Dir-2150 recovery boot simply always fails for me - the closest thread is this one
https://forum.openwrt.org/t/dir-2660-failing-to-flash/77917 - basically same problem.
From the serial line log - the closest among all attempts was this curl:
curl -v -s -F 'file=@DIR-2150A1_V106B01_unencrypted.bin' 192.168.0.1
give this:
=================================================
Check image validation:
Image1 Header Magic Number --> OK
Image1 Header Checksum --> OK
Image1 Data Checksum --> OK
=================================================[check_img_return:0]
image is unencrypto kernel images
*********** len: 1355, s->dataSize: 1355
But here it simply start to blink endlessly without any progress.
In fackt on Windows I was not even able to get this nice 'result' - although I've tried even Netscape browser, here is the typical result:
=================================================
Check image validation:
Image1 Header Magic Number --> Failed
=================================================[check_img_return:1]
=================================================
Check image validation:
Image1 Header Magic Number --> Failed
=================================================[check_img_return:1]
uboot magic err 0
=================================================[check_uboot_return:1]
*********** len: 940, s->dataSize: 940
*********** len: 1156, s->dataSize: 1156
So now - is there some practical way how to get back to original DLink firmware via openwrt cmdline ?
This is likely my 'best' revert attemp - booted DLink firmware - but non-functional rootfs.
[ 3.533372] mt7530-mdio mdio-bus:1f: MT7530 adapts as multi-chip module
[ 3.565766] mt7530-mdio mdio-bus:1f: configuring for fixed/rgmii link mode
[ 3.580363] mt7530-mdio mdio-bus:1f: Link is Up - 1Gbps/Full - flow control rx/tx
[ 3.584322] mt7530-mdio mdio-bus:1f lan4 (uninitialized): PHY [mt7530-0:00] driver [MediaTek MT7530 PHY] (irq=23)
[ 3.618554] mt7530-mdio mdio-bus:1f lan3 (uninitialized): PHY [mt7530-0:01] driver [MediaTek MT7530 PHY] (irq=24)
[ 3.641493] mt7530-mdio mdio-bus:1f lan2 (uninitialized): PHY [mt7530-0:02] driver [MediaTek MT7530 PHY] (irq=25)
[ 3.664585] mt7530-mdio mdio-bus:1f lan1 (uninitialized): PHY [mt7530-0:03] driver [MediaTek MT7530 PHY] (irq=26)
[ 3.687326] DSA: tree 0 setup
[ 3.695689] UBI: auto-attach mtd6
[ 3.702315] ubi0: attaching mtd6
[ 4.108210] ubi0: scanning is finished
[ 4.129720] ubi0 error: 0x804796b0: too large reserved_pebs 229, good PEBs 211
[ 4.144124] ubi0 error: 0x804795bc: volume table check failed: record 1, error 9
[ 4.158859] Volume table record 1 dump:
[ 4.166505] reserved_pebs 229
[ 4.172914] alignment 1
[ 4.178991] data_pad 0
[ 4.185074] vol_type 1
[ 4.191137] upd_marker 0
[ 4.197230] name_len 11
[ 4.203470] name rootfs_data
[ 4.211291] crc 0x56635c83
[ 4.219152] ubi0 error: 0x8047e524: failed to attach mtd6, error -22
[ 4.231886] UBI error: cannot attach mtd6
[ 4.241367] /dev/root: Can't open blockdev
[ 4.249598] VFS: Cannot open root device "(null)" or unknown-block(0,0): error -6
[ 4.264505] Please append a correct "root=" boot option; here are the available partitions:
[ 4.281147] 1f00 512 mtdblock0
[ 4.281161] (driver?)
[ 4.294203] 1f01 512 mtdblock1
[ 4.294214] (driver?)
[ 4.307220] 1f02 256 mtdblock2
[ 4.307231] (driver?)
[ 4.320239] 1f03 256 mtdblock3
[ 4.320250] (driver?)
[ 4.333256] 1f04 40960 mtdblock4
[ 4.333268] (driver?)
[ 4.346304] 1f05 13952 mtdblock5
[ 4.346316] (driver?)
[ 4.359323] 1f06 27008 mtdblock6
[ 4.359334] (driver?)
[ 4.372345] 1f07 32768 mtdblock7
[ 4.372357] (driver?)
[ 4.385387] 1f08 40960 mtdblock8
[ 4.385398] (driver?)
[ 4.398407] 1f09 6144 mtdblock9
[ 4.398418] (driver?)
[ 4.411431] 1f0a 8704 mtdblock10
[ 4.411442] (driver?)
[ 4.424644] Kernel panic - not syncing: VFS: Unable to mount root fs on unknown-block(0,0)
[ 4.441095] Rebooting in 1 seconds..
Also I've checked 'ethtool -s wan speed 1000 autoneg off' and this made WAN port unusable - switching it back to 100 restored functionality.