I've found that a good stout Panhard rod makes a pretty huge improvement. Telescoping shocks on their own usually don't... Here's a pretty good article on Panhard rods, etc.
https://www.britishv8.org/Articles/MGBRearSuspension.htm
As for whether or not the stock rear suspension was "pretty good to begin with"... well it's just a live axle on leaf springs! Not exactly an engineering marvel. I actually see big problems/limitations with it. For one, good quality replacement leaf springs are hard to find. The major supplier have major quality problems (such as mis-aligned spring eyes). The replacement springs aren't as durable as the original springs were; the last set I bought are sagging bad after about 10,000 miles. The supplier who made that set also left out the plastic anti-friction sheets that should have been between leafs, which is just plain sleezy.
Most people who do V8 engine swaps end up changing rear axles because the stock MG ratio (3.909:1) doesn't suit, and also many of us want limited slip differentials. The Salisbury axle isn't light! If it were lighter, ride quality would be better... so a narrowed Ford 8" pretty much beats it on all counts.
Where the leaf springs mount into the MGB floorboard isn't very stout. We find a lot of benefit to strengthening/stiffening that area (as Rover did on the MG RV8). Here's an article on that:
https://www.britishv8.org/Articles/MGB-Chassis-Reinforcement.htm
We also find that the MG leaf springs can't stand up to the torque our new engines have. This can be addressed with traction bars, but more and more often, we're upgrading to whole new rear suspensions. There are several excellent replacement/aftermarket MGB rear suspensions available, or you can make your own. Here are a couple examples...
The new Fast Cars three-link:
https://www.britishv8.org/Articles/2007-TechSession6.htm
A very nice home-made four-link:
https://www.britishv8.org/MG/MartynHarvey2.htm
(I've driven this one... at Nelson Ledges race course. SWEET!)
Hoyle IRS:
https://www.britishv8.org/Articles/Hoyle-Suspension-for-MGB.htm
https://www.britishv8.org/Articles/Hoyle-IRS-Anti-Sway.htm
Jaguar IRS:
https://www.britishv8.org/MG/EvanAmaya.htm
Nissan-based IRS:
https://www.britishv8.org/MG/ToddBudde.htm
This article shows a very lightweight rear axle/suspension:
https://www.britishv8.org/MG/DonColeman.htm
Another EXCELLENT, BRAND NEW, aftermarket four-link MGB suspension will be featured in the December British V8 Newsletter.