Hmmm, you need to read more about the Lancaster. The bomb bay was also strengthened and more powerful engines and different prop blades were fitted. All in all, this was quite a lot of modification to a standard Mk.I, so 'pretty much as any other Lancaster' isn't true.
As for performance, the following is taken from The Secret Years, Flight Testing at Boscombe Down by Tim Mason (Hikoki, 1998) extrapolated from Air Ministry files:
(1) "Unlike the Tallboy, the Grand Slam of 22,000lbs required extensive aircraft modification, more power and a considerable increase in weight."
(2) Tables of Performance record B.I (Special) PB592 as at a MTOW of 72,000lbs as having a max rate of climb of 500 ft per min to 10,000 feet, taking 50 minutes to achieve. Ceiling was 18,600 feet and its maximum speed was a true airspeed of 245 mph at an altitude of 16,200 feet. This data is whilst carrying a Grand Slam.
By comparison, from the same table, Mk.I W4963 with an MTOW of 63,000lbs has a max rate of climb 720 ft per min to 9,200 feet, taking 44 minutes. Its ceiling was 21,000 feet and with a maximum true airspeed of 282 mph at 13,000 feet.
So all round, performance of the Lancaster B.I (Special) was worse than standard Lanc Mk.I, which is precisely what I stated.
No mystery about it. If you look at the page you provided, then compare with the page below, you'll see that I am right in stating that a Mk.I (Special) with an MTOW of 72,000lbs has lower performance, again, so once more, you've gone ahead and proven yourself wrong with your own information!
http://www.wwiiaircraftperformance.org/Lancaster/Lancaster_I_PD435_Performance.pdf
Wot you posted.
http://www.wwiiaircraftperformance.org/Lancaster/Lancaster_VI_JB675_Performance.pdf