Quality, capacitance and voltage rating matter, dimensional size doesn't, so if you're really keen you could go for something like Mundorfs as a replacement though being more moderate Nichicon KG gold are a perfectly good power supply cap. You could probably go up to 6,800uF though it might not make a significant difference. Don't take the capacitance up too high though.
You could add a smaller 1/100th or even 1/1000th cap in parallel with the larger pair. The logic behind this is that even the power supply caps are part of a circuit and the smaller cap will allow higher frequencies to pass through the circuit where the larger caps might be falling away a bit.
