The performance won't depends on the computer size, but on the GPU/CPU installed.
Beside performance I would consider also monitoring (you can edit 8 hours a day on a 15" screen) and connectivity (I see only a USB port over there).
It'll have a Thunderbolt port for sure. Everything Apple has released since Thunderbolt's introductions from MacBook Air on up has one.
My guess is it will have quad core i7 as the other MBP 15" has. If they consider it a "bottom end" 15" MBP it might have dual i5. Airs so far only have Intel GPU as does 13" MBP whereas MBP 15" have Radeon.
This begs the question about MBP 13" though. If the Air and Pros get close we may see another round of consolidation.
Air 13" is currently dual i5 or dual i7 with Intel HD 3000 GPU
MBP 13" is currently dual i5 or dual i7 with Intel HD 3000 GPU
If that's truly a 15" "Air like" MBP you can probably say goodbye to the 13" MBP because the differentiating feature are likely to go away soon.
Unless something's changed in Apple's business model, they long ago gave up the confusion overlapping models.
Apple has had overlap with 13" MB and 13" MBP with the latter have some clear advantages.
I don' think they'll be product overlap at 15" so I really think we're on the cusp of another major change if the rumor is true.
It may well be a 15" Air and the site's second guessing the rumor is wrong. The difference between the Air and Pro will be the GPU and a Pro, even sans Optical Drive may have different internal cooling requirements due to the GPU.