To add some curiosity to this question, I figured something out in another thread:
https://forums.macrumors.com/threads/1103007/
Presuming I am correct about the chips (not verified).
Common
- 4 Threads
- 12 EU Intel HD 3000 Graphics
- 35W
- Dual Channel 1333MHz memory
- DMI 2.0
- 384MB Shared Memory for IGP
i5-2410M (base 13")
- CPU Base Clock: 2.3GHz
- CPU Turbo Clock (1C/2C): 2.9GHz/2.6GHz
- Cache: 3MB
- Graphics Standard Clock: 650MHz
- Graphics Max Turbo Clock: 1,100MHz
i7-2620M (best 13")
- CPU Base Clock: 2.7GHz
- CPU Turbo Clock (1C/2C): 3.4GHz/3.2GHz
- Cache: 4MB
- Graphics Standard Clock: 650MHz
- Graphics Max Turbo Clock: 1,300MHz
Based on these numbers. Due to the way Sandy Bridge works, the IGP has access to the same shared L3 as the CPU. It also can clock itself up to consume available TDP headroom if needed for graphics applications. This means that the graphics potential for the i7 13" is greater. Especially in applications that do not consume very much CPU. Games like Starcraft 2 which tend to use reasonable amounts of CPU will benefit less from this.
Best graphics performance for the 13" MBP can be had by matching SODIMMs in both slots for proper dual channel access, providing more cache for the whole chip, and purchasing the top-shelf CPU for maximum IGP turbo potential.
Edit: This appears to also be the case for the 15" models.
Common:
- 8 Threads
- 12 EU Intel HD 3000 Graphics
- 45W
- Dual Channel 1333MHz memory
- DMI 2.0
- 384MB Shared Memory for IGP
i7-2630QM (base 15")
- CPU Base Clock: 2.0GHz
- CPU Turbo Clock (1C/2C/4C): 2.9GHz/2.8GHz/2.6GHz
- Cache: 6MB
- Graphics Standard Clock: 650MHz
- Graphics Max Turbo Clock: 1,100MHz
i7-2720QM (best 15")
- CPU Base Clock: 2.2GHz
- CPU Turbo Clock (1C/2C/4C): 3.3GHz/3.2GHz/3.0GHz
- Cache: 6MB
- Graphics Standard Clock: 650MHz
- Graphics Max Turbo Clock: 1,300MHz
i7-2820QM (BTO 15")
- CPU Base Clock: 2.3GHz
- CPU Turbo Clock (1C/2C/4C): 3.4GHz/3.3GHz/3.1GHz
- Cache: 8MB
- Graphics Standard Clock: 650MHz
- Graphics Max Turbo Clock: 1,300MHz
In this light, the upgrade to the best model 15" makes a lot more sense. You end up getting a lot more clock when you take into account Turbo. It isn't just an extra 200MHz, it's an extra 400MHz even with all cores active. You get a decent bump in the IGP maximum speed as well. Not to mention the vastly better dGPU and other goodies.
The i7-2820QM BTO option looks less attractive. It is truly only 100MHz across the board, and a 2MB L3 cache increase.