Hi Teganc,
Seal Subscriptions here, a subscriptions app for Shopify stores (https://apps.shopify.com/seal-subscriptions). Increasing the API call rate limit most likely won't make much of a difference. This usually comes down to the actual implementation of the Shopify API in the app itself.
For example, in our Seal Subscriptions app, we have recently released an update to our code, which processes transaction incredibly faster. For example, our app has a feature, which allows customers to charge themselves at any time (if they want to) and the app processes that transaction a second after they click the button to charge themselves.
