The main router uses subnet 192.168.0.0/24, thats why wifi clients uses 192.168.1.0/24 as its required to use different subnets, but I also tried setting wifi client to 192.168.0.1 but same behaviour occur.
It does looks like something is blocking traffic, but since xbox360, a win7 box and Morphos netstack seems to communicate unblocked, what does these send to a router that Roadshow doesnt ?
There's not much left that could play a part. Say, could you try tcpdump again, with the options -s1500 and -vvv enabled (-s1500 tells tcpdump to read the whole Ethernet frame and not just the minimum required to make sense, and -vvv produces more output, decoding everything that might be even remotely interesting), and restart the DHCP negotiation?
It's possible that the Ethernet hardware address might not be sound. It's also possible that there is a filter rule set which permits only specific Ethernet hardware addresses to be used.
Or it might have something to do with the fact that Roadshow is 68k code and needs to be emulated.
All three are rather unlikely, but you never know...
And the different approach of setting a static ip seems to keep Roadshow in the dark aswell as pinging anything on the known network doesnt work, only Roadshow's ip.
Well, let's try something else. When your XBOX360 is online, pop into the network settings, write down the configuration currently in use (IP adress, network mask, router address, DNS server addresses), shut down the XBOX, then enter exactly the same network configuration information into the Roadshow interface configuration file that currently doesn't like DHCP.
It would look something like this:
device=sungem_eth.device
state=online
address=192.168.1.102
netmask=255.255.255.0
requiresinitdelay=no
The DEVS:Internet/routes file would look something like this:
default=192.168.1.1
And the DEVS:Internet/name_resolution file would look something like this (assuming that the router also acts as a DNS proxy):
nameserver 192.168.1.1
As for the aging setup aspect, its not limited to G speeds, you can setup two AC routers and brigded toghether they exchange data at ~600 Mbit/s in good conditions, with a gigabit port as in this Powerbook with Morphos, a remote desktop connection to a pc is bearly noticable being networked, you can watch videos or youtube with that 
600 MBit/s? You must be living far out in the country, with nary a bit of electromagnetic interference whatsoever ;-) I never even managed to 300 MBit/s out of two 802.11n devices placed less than 30cm apart.
