I have 8 different Grevilleas; have not grown 'Lava Cascade'. The groundcovers 'Coastal Gem' and 'Mt. Tamboritha' have been the slowest growers. The rest ('Moonlight', 'Peaches and Cream', 'Robyn Gordon', 'Superb', 'Ned Kelly', 'Fanfare', have all been fast growers. All have reached about the predicted size, so if the instructions say 6-10 apart, plant them 6-10' apart.

One thing about Grevilleas. They will sit there doing nothing for the first 3-6 months. Not an inch of growth, and you wonder what's going on. Then they explode with growth, very suddenly they are double the size.

They need water to get established, and we are at the start of summer, so keep an eye on them and water attentively to get them through their first summer, carefully if you have clay soil. They live longer and look better with pruning, so prune as needed. And remember no fertilizer. Zip, nada, none.

G. noellii, G. paniculata, and G.'Long John' can be added to the list of fast growers. Once those Grevillea get going they can put on some size quickly.

The ground cover G. crithmifolia is slower but it's also much more dense than other larger species

How wide is your border btw?

8 feet. But in the corners where this went it's wider than that even. I needed on that's a short height and wide width so I bought a few lava cascade and put them in already. Love them.