How could Apple make it less tall, lighter, more powerful?
XPS 15: 11-17mm (0.45-0.667")
XPS 13: 9-15mm (0.33 – 0.6")
13" MBP: 18mm (0.71")
15" MBP: 18mm (0.71")
MBA: 3-17mm (0.11-0.67")
rMB: 3.5-13mm (0.14–0.52")
The 15" rMBP has kept the same height - 18mm whilst improving it's battery 28% from the previous generation, and getting a 5% increase between the 2013 vs 2015 models.
The 13" rMBP has gone from 19mm to 18mm - whilst keeping it's battery fairly similar (74 -> 71.8 to 74.9Wh - improving it's battery 17% from the previous generation.
Every generation, the battery has been higher Wh, and there have been improvements from previous models.
This happening as the rMBP went from 19mm to 18mm.
The 13" 2nd gen MBP was 4.5lb (2.04kg), now it's 3.48lb (1.58kg) - 1 lb / ~500g lighter.
The 15" 1st and 2nd gen MBP were ~5.6 lb (2.5kg) and rMBP 3rd gen now 4.49 lb (2.04kg) - also ~1lb lighter.
Apple does seem to have a track record of being able to reduce weight, improve battery, whilst improving performance.
Will the CPU be less powerful than wanted by some? Yes
Will any dGPU be less powerful than wanted by some? Yes
Will it be an improvement on the current ones in terms of weight, size, and performance? I'd say likely. If it's not Town Hall (300 seats), but before WWDC - there's always Yerba Buena Centre (700 odd seats) (Flint seems a bit big)
Boring battery info
15" MBP Batteries 1st Generation
Early 2006, Late 2006, Mid 2007, Late 2007, Early 2008: 60 Wh
2nd Generation - Unibody
Late 2008, Early 2009: 50Wh
Mid 2009: 73Wh
Mid 2010, Early 2011, Late 2011, Mid 2012: 77.5Wh
Retina 15"
Mid 2012, Late 2012, Early 2013, Late 2013: 95Wh
Early 2015, Mid 2015: 99.5Wh