Cisco Catalyst 6500 switch capacity questions

We have a customer that requires redundant 10-GE uplinks from a Cisco Catalyst 6500-series switch to 15 closets.  The customer is hoping to implement a 6513.  I realize that the 10-GE line cards are oversubscribed on this platform (2:1 for the 8-port, and 4:1 on the 16-port modules), but I have several "deeper dive" questions.

We've received information indicating that the 6509 is better suited to handle this level of 10-GE density than , and I'm wondering why.  Here's a link indicating that the 6513 can only handle 20 10-GE (XENPAK) ports, and that the 6509 can handle 32.

http://www.cisco.com/en/US/prod/collateral/modules/ps2797/ps5138/product_data_sheet09186a00800ff916_ps708_Products_Data_Sheet.html

These switches handle non-XENPAK line cards, however, including 8-port and 16-port boards w/ X2 modules.  I realize these are oversubscribed at 2:1 and 4:1, respectively, but the Cisco Product dictates that you can achieve a total of 84 such ports in the 6513 chassis (compared to 130 for the 6509).

http://www.cisco.com/en/US/prod/collateral/switches/ps5718/ps708/product_data_sheet09186a00801dce34.html

So, my questions:

1) Why would the 6513 would top out, from a design perspective, at less than 30 10-GE ports if we were using X2 modules instead.

2) If we're using Supervisor 720 engines (720-Gbps backplane) and this provides 48-Gbps per slot, why again would there be such a limit?

Thanks in advance for hopefully somewhat-detailed answers to these two...