When launching a significant sub-product (e.g. an enterprise version of an existing product) how do you strike the right balance of enabling your sales team without bombarding them with dizzying details?

It depends on how radically different your Enterprise solution actually is and how unfamiliar it will feel to Sales. Something else to consider, is this a new tier or a totally new product?  

Often your Enterprise solution isn't radically different from your Standard product, especially at launch. Also, if your sales team has come into contact with enterprise customers they're probably already clamoring for the enterprise functionality. If this isn't the case though and let's say you've acquired or built a more standalone enterprise solution and are trying to train your SMB sales team on it, I'd start slow and then ramp up. Make sure the team really understands the enterprise pain points the new solution is meant to solve. What's the value proposition? Key selling points? Starting high level this way, you can ramp your team up slowly on all the details. With Enterrpise sales, your volume is probably fairly low especially in the beginning so you can have a Solutions Architect or even PMM help out with demos until the Sales team feels more comfortable.