I ran 8sp shifter and LX derailleur on my Mongoose 27+ and it was much better. After a couple weeks I spent $31 + shipping for an 8sp freewheel from Universal. The shifting was better than the limited-out 8 shifting 7. The main reason I got it was the stock 26T big cog made for a pretty steep low gear coupled with gigantic tires.

https://www.universalcycles.com/shopping/product_details.php?id=49867&category=1665