Im not sure what miracle gro soil is like, never used it. I have used perlite however, its ok but I prefer vermiculite because when perlite comes to the surface its very lightweight and gets everywhere.
But Im not sure this plant is in dire need of repotting. Unless its showing ill effects above soil Id wait till spring.

I wouldn't be in a big hurry to repot it now, especially if you're pleased with how it's growing. I think potting up to a 6" pot and being attentive to water needs (with an emphasis on not over-watering) is a sound plan, though. Then, when summer comes around, you can get into the roots if you wish. The plant should respond as favorably as light will allow to the potting up, but it certainly wouldn't set if back unless you're heavy-handed on the watering can. ;-)

Spiders like a fast draining soil that doesn't threaten roots by remaining wet for prolonged periods. Where you live, you should have good access to materials like fir bark, Turface, Axis/Play Ball, and others you can use to make a very free-draining soil your plants will really appreciate. You'll even be able to eliminate the perlite, which can add to the amount of fluoride the plant is subjected to and is sensitive to - not a huge plus, but every little bit helps. Good growing is about identifying the factors that limit your plants and acting to eliminate or reduce to the greatest degree possible their effects.

Al

Al, I never realised that perlite had flouride in it, interesting!

It truly is interesting to note the number of useful tidbits you can pick up here at GW just by being here. I learn interesting things, or discover new things I want to look into before I post, every day.

The amount of fluoride in perlite varies by it's source; and for plants sensitive to compounds of fluorine, rinsing perlite well in a strainer or wire colander can help reduce the amount of fluoride that makes it to the plant.

Spider plants prefer low TDS/EC (fertility) levels, which is why soils you can water/flush at will and fertilize frequently at low doses go a long way toward preventing spoiled foliage. Keeping the humidity up is very helpful, but keeping roots working efficiently so they can easily move water to distal parts, leaf tips and margins, is the biggest factor affecting the plant's appearance.

Al

I don't know how you don't have brown tips with the roots like that. Yesterday I unpotted one and was shocked by how much the roots had crammed the pot in about 6 mos. I don't use perlite and can't do much about it getting tap water sometimes, and while it's inside for winter. Plenty of sun here in winter, though, I did repot it, in the way I've done these for a LONG time.

Before:

During:

Ready to go back in its' pot.

Purple, is that an all green chlorophytum?

yes.

Ive never seen one in the flesh. Theres also a different coloured one, variegated but exotic looking.
Havent had a spider for years! My cats would love one, lol