A system with four HDDs is different from a system with one: intake is more restricted and heat production is heavier.
A system with a bottom-intake PSU is different from a rear-intake one: pressure and airflow are generated differently as fan size and orientation are different.
And then there are the individual differences from different cabling, drive and PSU orientation and pressure sealing. Some may have to have the above hatch open more than others, lessening pressure generation through this fact alone.
What I meant to say was that in theory it happens exactly like is said: there will be no heated exhaust returning into the case through the rear vent that is now blocked, and there will be more negative pressure as a result of a more sealed environment -- the theorized results are true. The results of these
, such as achieved cooling power and silence, vary by system because of the factors mentioned above.
A system with a single HDD, a PSU with a 1000+ RPM fan and a completely sealed compartment(intake and exhaust excluded) will offer sufficient performance for both HDD and PSU cooling to work. Heat is extracted and PSU fan won't have to ramp up to cool the unit. Both PSU and HDD work at cool temperatures.
A system with three HDDs, a PSU with an <800RPM fan and a partially sealed compartment will gain from the modification, but the benefit may fail to realize in concrete terms of cooling power or silence -- there is just too much heat and too little extraction. The fan may not ramp up as the PSU can take the heat, but HDD cooling won't be much better than without the modification. PSU will work at hotter temperatures than before, HDD cooling may see a change if PSU heat spreads in the compartment or negative pressure is generated better.
A system with a full HDD rack, a passive PSU and an open compartment will suffer and most likely become unstable, gaining no benefit from the modification.
What happens with real systems between these is anybody's guess, and for the owner to find out.