I'd also say it unusual to see significant heat but a slight warming up i would say would be normal. Is this a functioning design?
C2 Looks unusually placed, its function isn't too obvious to me. Its normal to have a small RC on the current sense which is your C29 & R7 but C2 i struggle to identify its purpose.
I'm not familar with the UC3843 but I also find the shorting of the FB to ground quite an interesting idea i've not seen before. This will pull the error amplifier 5V making the COMP pin a fixed current source but eliminates the error amplifier functionaly in the IC to instead be created from the TL431 & opto.
T6 looks to be providing an Auxilary VCC via the 5R1 resistor obviously the higher the voltage on this the more heat you'll be disipating.