The performance stats I'm familiar with have the JU-88A being faster than 270mph and the JU-188 being slower than 325mph.

For example, HTC's own performance curve shows the JU-88A maxing at around 290mph TAS.
Another consideration is max internal bombload. The JU-188 is no improvement here, whereas the Do-17 could carry 2500kg of bombs internally, compared to the 88/188's 1000kg internal load. More payload internally means less drag, so a faster bomber, a more capable machine.
The one thing I'll give you is that the 188's defensive armament was frequently an MG-151 in the dorsal position, rather than the Do-217's MG131.
