Under the assumption CR's rumor is true

Difference of v2 as compared to v1

- More lens elements and groups

- Differing Focus adjustment

- Shorter focusing distance by 0.82m/2.7ft

- Zoom system possible twist-type rather than push-pull

- Larger Diameter x Length, Weight 2mm x 4mm; 190g

- 2 more stops of IS

- 16 year gap between the introduction of the v1 & v2

- $500 more

- Inclusion of IS Mode 3

http://www.usa.canon.com/cusa/consumer/standard_display/Lens_Advantage_ISVersion 1 vs 2

v1

Focal Length & Maximum Aperture 100-400mm 1:4.5-5.6

Lens Construction 17 elements in 14 groups

Diagonal Angle of View 24° - 6° 10'

Focus Adjustment Rear focusing system with USM

Closest Focusing Distance 1.8m/ 5.9 ft.

Zoom System Linear extension Type

Filter Size 77mm

Max. Diameter x Length, Weight 3.6" x 7.4", 3.1 lbs. / 92mm x 189mm, 1,380g

2-stops of IS

Introduced 1998

Price $1,699

v2

Focal Length & Maximum Aperture: 100-400mm 1:4.5-5.6

Lens Construction: 21 elements in 16 groups

Diagonal Angle of View: 24º-6’10’

Focus Adjustment: Inner focus system / USM

Closest Focusing Distance: 3.2 ft. / 0.98m

Filter Size: 77mm

Max Diameter x Length, Weight: 3.7 in. x 7.6 in. / 94mm x 193mm; 3.46 lbs. / 1,570g

4-stops of IS

Introduced 2014

Price $2,199