Likely top speeds will be:
- 4 Cyl Diesel: 194 Kph
- V6/8 Diesels : 209 Kph (225 Kph with dynamic pack)
- P400e(Hybrid): 220 Kph
- S/C V8: 225 Kph (250 Kph with Dynamic pack) - I highly doubt this will happen
- In line 6: Likely 220 Kph - certainly no more than 250 Kph
This is all based on the Disco 
Series 5 
(L462)/ RRS 
Series 2 
(L494) top speeds (Disco 
(L462) only comes in SD4/SD6 Diesel). I highly doubt LR will want a Defender going any faster than an RRS 
(L494).
Top speeds are determined either by limitation of engine RPM vs final drive ratio (diesel) or limiter for Petrol versions (RRS SVR tops out at 283 Kph).
The different engine types determine how quickly you get to the top speed.
Obviously, this would be for a road biased version
 of the new defender(in Dynamic guise with low profile 22" tyres and high power engine)... who knows what they 
(Land Rover) would limit an off road biased version to
 (if one were to be made with larger tyres that have a taller sidewall etc.).