A thule box of similar dimensions, gets three holdalls in plus 6 nets and three landing heads(not in stink bags) So You should be able to get just two Prestons in, best take the holdalls with you to your nearest stockist to check.
One drawback to any roof rack is the additional drag causing increased wind noise and higher fuel use. Another can be forgetting the headroom increase 😮
The benefit is more room inside the car and if you can get your nets in too then less stink in the car. Assuming you’re not giving a lift to Jonny Fartpants.