According to this site
http://www.aeroflight.co.uk/types/germany/junkers/ju_88/Ju_88_nf.htm
the nightfighter late versions of the Ju 88 could actually reach speed in the 630 Kmh range (see G7b) with a more than respectable cruise speed of 560 kmh.
The G7c, with the new covered aerials could even had a bit less drag and be a bit faster.
Junkers Ju 88G-1
Role: Night-fighter
Crew: Four
Dimensions:Length (excluding radar) 47 ft 8.5 in (14.54 m), (including SN-2 aerials) 54 ft 1.5 in (16.50 m); Height 15 ft 11 in (4.85 m); Wing Span 65 ft 7.5 in (20.0 m); Wing Area 586.63sq ft (54.5 sq m)
Engine(s): Two 14 cylinder, air cooled, BMW 801D-2 radials of 1,700 hp (1268 kW) each.
Weights: Empty Equipped 20,020 lb (9,081 kg); Normal Take-off 28,870 lb (13,095 kg); Maximum Take-off 32,385 lb (14,690 kg)
Performance: Maximum level speed 356 mph (573 kph) at 27,890 ft (8,500 m) with SN-2 but no upward-firing guns, 342 mph (550 kph) at same altitude with 'Schräge Musik' installation; Service ceiling 29,000 ft (8,840 m); Normal range 1,553 mls (2,500 km); Maximum endurance on internal fuel 4.75 hours.
Armament: Four fixed forward-firing 20mm MG 151 cannon in ventral tray with 200 rounds each and one flexible 13 mm MG 131 machine-gun at rear of cockpit with 500 rounds. Optional 'Schräge Musik' installation in upper fuselage with two 20 mm MG 151 cannon firing obliquely forward
Junkers Ju 88G-7b
Role: Night-fighter
Crew: Three
Dimensions:Length (excluding radar) 47 ft 8.5 in (14.54 m); Height 15 ft 11 in (4.85 m); Wing Span 65 ft 7.5 in (20.0 m); Wing Area 586.63sq ft (54.5 sq m)
Engine(s): Two liquid cooled, 12 cylinder inverted-Vee, Junkers Jumo 213E of 1,725 hp (? kW) each.
Weights: Empty Equipped 20,503 lb (9,300 kg); Normal Take-off 28,885 lb (13,100 kg); Maximum Take-off 32,353 lb (14,675 kg)
Performance: Maximum level speed 270 mph (434 kph) at sea level, 363 mph (585 kph) at 33,500 ft (10,200 m), 389 mph (626 kph) at 29,529 ft (9,000 m) with MW-50 emergency boost; Cruising speed 348 mph (560 kph) at 29,529 ft (9,000 m); Initial rate of climb 1640 ft/min (500 m/min); Time to 30,185 ft (9,200 m) 26 min 24 sec; Service ceiling 32,810 ft (10,000 m); Normal range 1,398 mls (2,250 km); Endurance (at maximum economical cruising speed) 3.72 hours at 29,800 ft (9,083 m).
Armament: Four fixed forward-firing 20mm MG 151 cannon in ventral tray with 200 rounds each, two fixed oblique upward-firing 20mm MG 151 cannon in dorsal 'Schräge Musik' position with 200 rounds each, one flexible 13 mm MG 131 machine-gun at rear of cockpit with 500 rounds.