I just meant to try it to see if your hesitation improves. I can't say I've seen a problem with a car going from closed loop to open loop control when you press the throttle but I'm not saying it doesn't exist because logically it could. If you disconnect the sensor it can't do closed loop anymore. So if you disconnect it and the hesitation has gone then you know the lambda is the problem. Otherwise your lamabda might be wrong but you've got another problem as well, (ignition, fuel, air).
I run my PFC in open loop as the closed loop control keeps going rich/lean whilst in open loop its more stable around 14.7. This took a lot of fettling though and a standard map wouldn't necessarily be as accurate. When a Lambda sensor is failing it can make the situation worse rather than better which is why your emissions can go completely to pot.
Being honest its been a few years since I've played with the standard ECU and I can't remember what the failure mode is for no lambda but I'm sure a quick search will help. Besides, it'll take as long to disconnect the sensor as it will to find the info in the manual ;-)