You do not need to spend extra money on soil mixes with slow release fertilizer for an established shrub that is doing well. Regular top soil, compost or composted manure will do the job.
If you look at what's in the mixtures they are pretty much the same: forest products, sphaghnum moss, manure and a slow release fertilizer. Unless a shrub is showing signs of a nutrient deficiency all of it's needs can be met from the existing soil. Top dressing with compost or composted manure on a yearly basis is all that should be required.
Slow release fertilizers are a great idea for annual baskets. Annuals can use a lot of energy in flowering.
There are only a few situations where this answer could be reconsidered:
- acid loving plants such as blueberries, rhododendrons do better in an acid soil. If you do have acid soil topdressing with peat moss mixtures and acid fertilizer may be required.
- if your soil is of a special type like caliche then a soil mix with fertilizers may assist
- when growing plants in a greenhouse a peat based soil mix works better for a number or reasons ( sterilized, lightweight, readily available)
Edit: Composted cow manure is a good top dressing. You could apply a layer up to an inch thick twice a year, spring and summer.