If you've got nothing else running off the 5V rail provided by the Arduino's regulator, then 11.3V would be good.
However you probably have a few 5V analog sensors and they really should be powered from the same rail as the Arduino, so that the voltages are comparable. (Don't try to measure a sensor powered from an unrelated power supply unless all you are doing is measuring that power supply.) I don't expect that 5.3V will make any difference to the Arduino if you bypass the regulator.
I have an arduino mega, a 3.2" tft lcd, an ethernet shield, a 4*4 keypad, some IR sensors (tcrt5000, using 2 for now), a DHT22 and a GY-63 MS5611 module. I think that those would be powered from the arduino.
I have also 3 servo motors (sg-5010), 5 small dc motors (don't have them all for now), a 5V 8-Channel Relay interface board, some LED (about 3 or 4). I think i must power them from the psu.
If i remembre correctly i read that all the components should have rhe same ground, is that correct? if not, should i use a fuse for each circuit? (there are many black wires , are they concidered different grounds or just the same?)
Do a search of ATX PSU conversion and you'll get a ton of tutorials on how to convert an unused internal PC PSU into a mnoce lab bench one. I am almost finished doing mine...