At least you have much more reliable engines in the A5 model...there are accounts of 177s reaching nearly 700kmh in flat dives to escape the attention of prowling fighters...
Not sure about the reliability, but the flat dive thing is well accounted here:
"While Steinbock was unsuccessful, the He 177 did achieve some successes. They typically carried two 1,800 kg (3,970 lb) and two 1,000 kg (2,200 lb) bombs. Climbing to 7,000 m (22,965 ft) while still over German territory, the He 177s approached the target in a shallow dive, each aircraft throttled back, the pilot putting his aircraft into a gliding descent to take it across the bomb release-point at about 4,500 m (14,760 ft). After releasing the bombs the pilot re-opened the throttles, but continued the descent at approximately 200 m (656 ft) per minute. The bombers typically re-entered German airspace at an altitude of 750 m (2,460 ft), and headed back to base. By such means, the He 177s were able to keep up speeds of about 600 to 700 km/h (370 to 430 mph) during their withdrawal phase. The higher speed and constant change of altitude made interceptions difficult, increasing the survivability of the aircraft, but decreased accuracy. "
http://en.wikipedia.org/wiki/He-177#Operational_history