From what Slaists posted on twcenter, the experience of reinforcements varies with the rate of replenishment. If you replenish slowly (e.g. in an undeveloped castle), your reinforcements tend to have higher average experience. If you replenish fast, your 5-exp unit will soon drop down in quality.
However, I think this is largely irrelevant given that you seem to pay full support cost for a one man unit. Given that, it seems much better to consolidate depeleted units after loses and just recruit new ones. That will save you money and give you an elite force of veteran units.
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