Update:
-gglabs unit is definitely dead,contacted the seller and waiting for a response;
I cannibalized around 20 HY53C464LS-80 from VGA cards (the Hakko FR-300 is an AMAZING desoldering gun), removed all the old DRAM and replaced them with the cannibalized parts. Supposedly these parts are 80ns, so more than fast enough for what i wanted.
But result is pretty much the same as before at 10MHz.
So i tried tapping the 28.6 crystal on the board again for 9.54MHz speed and now the system worked fine. Games, programs everything working just great. So i tried to get the system faster by running SETUPSL /A for 0 waistate memory. I saved the settings, rebooted, ran checkit and...weird graphic corruption again! WTF? So i ran SETUPSL /A again and restored BIOS defaults. I was sure that this would solve the issue but it didn't. My guess is that that serial EEPROM surely doesn't hold data very well now. I checked the datasheet and it states that it should be good for 40,000 write cycles and 10 year data retention. This is surely not a very high endurance serial EEPROM for sure, and it seems that if i write to it when the system is running at a speed of anything greater than 8MHz it just craps out. I ordered a pin compatible ATMEL 93C46E serial EEPROM to replace the other.
I also took a look into the datasheet as well, and noticed that besides the CPU, 2 other IC's will take the CPUCLK generated by the buffer blue. One of them is a 74 series logic chip a 74LS175 flip flop and a PAL16R4A. From what i could understand from the datasheet, the PAL16R4A can take clock speeds up tto 25MHz so that shouldn't be an issue. The 74LS series chips are low power versions of the 74S versions which were in other way very fast but with high power consumption. The LS are exactly the opposite, in fact the same speed as the original 74 series released in 1966. So i ordered a 74ALS175 flip flop to replace the old one although i have no idea if this is the main issue.
Some good news otherwise is that the case keenerb sent me is now in my hands. Spent a couple hours at customs to pick it up, but i'm happy and surprised how small it is:
In any case i can always try to find a TL motherboard which i was told fits the SL case. The TL has more or less the speed i would like.