Does enabling auditing on the Subscription Management header or product impact performance?

Yes, performance is impacted if you enable audit on a subscription management header.

An audit framework goes by the transaction and not by the fields. If multiple fields are updated in the same transaction, only one row gets populated in a shadow table per product. But if all 6000 product lines are updated, it results in a new row in the shadow table for each product per transaction. This could become an overhead that you might want to consider.

Also, audit framework does row by row processing. If you enable audit for an object example subscription, a new record is created for each update for the subscription. This is to ensure proper tracking of any updates for the object. If there are frequent updates for subscription or products, a new record will be created for audit each time an update happens.