I notice on another similar product the max per socket is also 60watt, so it seems to be case across the board on these products.
To get the best out of a big cfl you either need to hang it vertically ie hanging down and have it between your plants like a vertical grow or with a wing type reflector hung vertically to direct the light, so I'm not sure this adapter would be ideal if you're set on the big cfl route.
The truth of the matter is you will struggle to get big buds from cfls, you will also find heat is an issue as they generate a lot of it.
They're also inefficient by virtue of their design, light is thrown in every direction rather than directed down towards your canopy where you want it.
Another thing to consider is that a 300w CFL is around 18inches long, that's about 45cm long with about a 7inch / 18cm diameter, in short they are huge like really really big, certainly larger than any buds they will grow.
It's also worth knowing that they are very fragile and any bump against them will render either the whole thing dead or partially dead.
I'd focus on either hps or led if bud size is one of your primary goals
The cheapest way is with the 1 to 7 converter from amazon with 7 x 13w (1521 lumen) Phillips led light bulbs.
For around $35 you would then have 10647 lumen in total, around half that of a 300w cfl but much more efficient as you are only using 91 watts.
Not only that the light is focused exactly where you need it, rather than at the ceiling etc.
You can add another one when you have more money or buy two converters and add bulbs as and when deals come up