NO, cable management is not the only culprit with heat issues; and if you cannot do good cabling in a large case like K380, I doubt if you can do any better in the Master case 5; Don't waste money;
Let's try to address your heat issue:
1. If you suspect cabling, buy some rubber bands (cable ties, if available) and bind the floating ones, put most of the unused cables next to the PSU on the floor
2. For good airflow, buy 120mm fans and arrange like this: front intake, bottom intake; side outflow, top outflow, rear outflow; if you are short of fan mounts, get fan splitter and connect similar ones to the same mount
3. Relocate your CPU to a place that gets air movement
Notes:
1. Poor thermals can be addressed with air flow
2. Your build details like which CPU, GPU, PSU etc., will help greatly in identifying if you need addl, cooling