A combined Westpac and St George would have market leadership in several of the most important business banking product segments, according to a research note issued yesterday.The banking industry research group East & Partners said the merged group would have a share of about 25 per cent of overdrafts, about 22 per cent of term cash deposits and about the same level of at-call accounts.East & Partners' estimates are based on its surveys of what it calls the corporate banking market, which covers companies with turnover of $20 million up to $340 million a year.East & Partners analyst Rob Morgan said those shares would give the merged group market leadership. Morgan: "There are a lot of other product segments but those ones are key. The bank will be hoping to gain as many market leading positions as it can to use as a positioning tool in the market and with shareholders."Another research report published yesterday suggests that the retail customer base of the merged group would be significantly smaller than Commonwealth Bank's.Roy Morgan Research (which also uses survey data) said there was a 14.4 per cent overlap between St George and Westpac's retail customer bases. Taking this into account the combination of Westpac's 3.34 million retail customers and St George's 1.98 million would result in a customer base of 5.04 million.Commonwealth Bank has 6.88 million personal customers, according to Roy Morgan. On a state-by-state basis Commonwealth has 2.3 million retail customers versus 1.9 million for Westpac and St George in NSW, Commonwealth has 2.02 million versus 1.1 million in Victoria, Commonwealth has 1.2 million versus 756,000 in Queensland and Commonwealth has 554,000 versus 472,000 in Western Australia.The only state where the merged group would have a bigger retail customer headcount is South Australia, where it would have 460,000 customers versus 385,000 for Commonwealth.